This is where things get a bit complicated. Essentially, DPS figures suggest that while Scourge Strike is once again king, Reaping still doesn't become truly effective until you're in tier 10 armor, namely until you have the 2 piece tier 10 bonus. Below that, you're better off just firing off two extra Blood Strikes every round. Also, with Unholy Blight nerfed yet again, Morbidity is no longer amazing for single-target DPS, but is still very useful for AoE damage, which means that some unholy DPSers will actually keep two DPS specs, one for AoE and one for single target fights.
To start, here's a base talent build that doesn't use reaping. Once you have your 2 piece tier 10, you can go ahead and swap 3 points of out of a talent for Reaping. Dark Conviction is probably the smartest place to take those points. If you'd prefer to do more AE damage, use the build linked above, but take 2 points out of Necrosis (which is much stronger for single-target damage) and put them in Morbidity. That will allow you to fit Death and Decay into your rotation more often.
It's also worth noting that blood is currently the best secondary tree for an unholy build. Since Scourge Strike now does much of its damage physically, you'll find even Black Ice just doesn't buff unholy as much as blood talents do.
7. Talent overview
This section will cover everything in the talent build linked above, as well as the optional "switch out" talents for maximizing your DPS at various gear levels and roles.
Vicious Strikes - This ups the damage potential on your main strike considerably. Take it.
Virulence - While you don't do as much magical damage as you once did as unholy, this still helps make sure stuff like Icy Touch and Death Coil consistently hit. Take it.
Epidemic - This is one of the cornerstone skills of the unholy death knight. Whether you have reaping or not, you'll need this skill for the most efficient rotations possible, especially since losing diseases is especially catastrophic for unholy damage and utility.
Ravenous Dead - Not only does this buff your ghoul, but the extra strength helps you as well. This is a very considerable damage buff, and not to be missed.
Morbidity - This is actually pretty lackluster for single target damage, or at least not as good as other options, but if you're doing AoE damage on relatively long lasting AoE fights, a couple points here allows for you to fit Death and Decay into your rotation that much more easily.
Outbreak - This is a major damage buff to your main attack. It's a no brainer
Necrosis - Necrosis is a passable single target damage ability, and certainly better than most other stuff you could spend the points on, but it's also less potent than it appears to be, as it generally does not get any extra bonuses to its damage from talents, skills, or debuffs. When you get to AoE damage potential, it's a no brainer to take points from here and put them into Morbidity.
Blood-Caked Blade - This proc ends up doing quite a decent amount of damage when it hits, especially with 3 diseases. It's well worth the points.
Night of the Dead - You'll need this talent to get your perma-ghoul, which is one of the cornerstones of an unholy death knight. Even if you didn't, that 90% AoE damage reduction makes the whole idea of a perma-ghoul viable. Grab it.
Unholy Blight - Poor neglected Unholy Blight is a shadow of its once great glory as an AoE 4th disease runic power dump, but it's still worth taking, since it's a decent 1 point damage boost for your runic power dump. Still, if any talent gets a full revamp in Cataclysm, I hope it's this one. It just doesn't feel flashy enough to be a 21 pointer.
Impurity - This offers a pretty sizable damage boost to spells and diseases alike. It's definitely worth the points for any unholy DPSer.
Dirge - If you're bringing your A game, you'll need this talent to fire off as many Death Coils as possible. Don't let your runic power sit unused or consistently sit at 100 though, or you'll essentially be wasting these talent points.
Master of Ghouls - One of the signature abilities of an unholy death knight, this is another no brainer. You get some extra utility and damage out of your ghoul, as well as the ability to pull him out of sticky situations.
Reaping - One of the roller coaster talents of the unholy tree, this has been alternately essential or a waste of points, depending on the current status of Scourge Strike. Right now, it's sort of in the middle. DPS testing suggests that it's non-essential for tier 9 and lower DPS, but once you're in tier 10 gear, it becomes better again. If you have 2 piece tier 10, or if you just want to take Reaping now so you don't have to respec later, the best place to grab the points is from Dark Conviction.
Desolation - This is a nice 5% additional damage, but it's also a bit of a pain in the neck since it means you have to be tight enough with your rotations to be hitting a Blood Strike every 20 seconds. Still, if you're bringing your A game, this is a powerful talent.
Crypt Fever - Not only does this give your diseases a hell of an extra kick, it's a 3rd disease, so it's pretty much mandatory. Take it.
Bone Shield - Not only is it a quick 2% damage boost for one talent point, but if you do draw aggro, it makes you much more likely to survive until the tank can peel the mob back off of you. Take it and remember to keep it up at all times. Whenever you have some downtime and Bone Shield is off cool down, recast it.
Scourge Strike - Scourge Strike has had a bumpy road, but in Patch 3.3, it is back to dominance in PvE DPS. Take it, use it, love it.
Ebon Plaguebringer - This is a large part of the raison d'etre of an unholy death knight. It's an amazing, near essential buff for any raid group, and it's better than the warlock and moonkin equivalents because it can be so easily spread to multiple targets. Combine that with the fact that it buffs your own spell damage, and its a no brainer to take it.
Wandering Plague - This is a very nice contributor to unholy's legendary AoE damage, but it's also very useful for single target builds since it also adds extra damage to the original target, thus giving your diseases the effective ability to crit. Thus, it's essential for just about any unholy DPS build.
Rage of Rivendare - Not only does this ability supply you with a nice chunk of expertise, bringing the amount you need from gear down to 21, but it provides a ton of extra damage on targets with Blood Plague. It's incredibly essential for damage.
Summon Gargoyle - The Gargoyle is the runic power dump that was so powerful, it had to be switched from 21 points to 51 points. It certainly does provide a lot of extra DPS when used often and properly. It takes a snapshot of your stats when its summoned and uses those for its own stats, so make sure you're as buffed as possible when you summon it, including temporary stat buffs from trinkets and Heroism/Bloodlust. Of course, temper that with the knowledge that if you wait for the perfect moment to summon every single time, your DPS will suffer as the gargoyle stays locked away.
Butchery - This talent can be good for getting enough extra runic power for a Death Coil or gargoyle every so often, especially if you're chain pulling.
Subversion - Not only does this keep you from pulling aggro, the critical strike chance applies to Scourge Strike. It's worth the points.
Bladed Armor - It's very rare that you'll be wearing anything but plate armor, so this talent ends up giving you a lot of extra attack power, which will boost all of your damage.
Two-Handed Weapon Specialization - As unholy, you will be using a two-hander. 4% extra damage from everything you do with that two-hander adds up.
Dark Conviction - This does add a pretty decent amount of damage for an unholy death knight. It's more useful for AoE in that it allows more triggering of Wandering Plague. If you need to pick up Reaping to maximize your single target damage, this is the best place to take points though.
8. Leveling as unholy
Unholy is certainly a more than viable leveling tree. It doesn't have quite as much self-healing or ability to solo elite quests as blood, but it has that superior AoE ability, as well as some fun tools such as Bone Shield and the perma-ghoul. Talent wise, just go ahead and work up the unholy tree first, as none of the secondary talents over in blood are strictly needful for solo leveling.