Dislodged Foreign Object
This is by far the best warlock trinket in the game right now. Even the normal version beats the heroic versions of others to a bloody pulp. The only downside to using it is the uncomfortable feeling of wondering what the hell it is and where it got dislodged from. Rotface doesn't look the most sanitary of beasts but he is one of the easier bosses in Icecrown Citadel which makes this trinket fairly accessible. The downside is that everyone and their dog wants it.
Charred Twilight Scale
Currently only on the PTR this is set to become the new best in slot. The heroic Dislodged Foreign Object still trumps it however. Working in pretty much exactly the same way as the Abyssal Rune this is a low maintenance trinket (doesn't need to be triggered) that will nicely boost your damage even without you coinciding events with its procs.
I love this trinket. While having proc-based and on-use trinkets means you can work some well-timed magic I prefer the stability of a flat, consistent buff. This trinket offers much the same bonus as the Illustration of the Dragon Soul but with a nice big crit bonus as well. What really makes this trinket stand out is its availability -- dropping from the loot-boat in ICC-10 makes this a pretty easy find.
Phylactery of the Nameless Lich
Having a huge spell power buff but fairly long internal cooldown means that this trinket is very good but makes your damage more spikey. If you can make good use of those spikes then you can do good things with this trinket. Being triggered by a DoT pretty much ruins this trinket for mages, greatly increasing the change of being able to get your hands on it. On the flip side you need to be able to kill Sindragosa which can put it out of the reach of more casual players.
Reign of the Dead/Unliving
Before Icecrown Citadel opened up this was the BiS item to be seen with. Interestingly this proc doesn't buff you but simply adds in its own damage. Still a very powerful trinket but unless you have the heroic version then the ICC ones (above) will all be better. TotC-25 is a nice quick and fairly easy instance now so this is a pretty accessible trinket as long as you can convince people to go back there.
Flare of the Heavens
Another great proc-based trinket but if you don't already have it then I wouldn't suggest trying to get it. Few people are interested in Ulduar any more and convincing people to do General Vezax the hard way for your trinket might be a hard sell.
Maghia's Misguided Quill
As I mentioned in the article about Emblems of Frost, this trinket is hugely powerful but only if you need all the hit. If you don't (and most people really don't) then it's pretty poor. That said, if you have had no luck with drops and have a ton of emblems going spare then you could do worse. The on-use ability is good and it does free you up to choose gear upgrades you might otherwise reject for fear of dropping below the hit cap.
Illustration of the Dragon Soul
I was very lucky to get hold of this trinket quite early on and kept it right the way into ICC. You can "charge it up" before a fight (rank one life tap) so you maintain a flat 200 spell power buff the whole time. Given that Sartharion is still interesting to people as he's a quick zerg kill and drops a drake you might get chance for this item but it only drops in the 25 man version, so it can be tough.
Eye of the Broodmother
Pretty much any Ulduar ten man PUG can bag you this ocular beauty. Very nice trinket in its day and it will still tide you over well.
Glowing Twilight Scale
The PTR tells us this is the healer version of the Charred Twilight Scale from the Ruby Sanctum. Please don't need on this if a healer does, it won't do you anywhere near as much good as it would them. That said, if you don't have a decent trinket and happen to see this drop AND nobody wants it then a 190 spell power buff is better than an Abyss Crystal.
Purified Lunar Dust
Another healer trinket and not worth the cost. However, given the highly unlikely situation of your having emblems to burn and no better trinket then the flat 179 spell power isn't bad.
As with the other healer trinkets this one is mostly wasted on a warlock, so pass for a healer. Again though, the spell power is good if you have nothing better.
Scale of Fates
Another great warlock trinket in its day this one is probably not going to be something you stumble across very easily any more. Good for bit of burst when required though.
While being right down on the list this is probably the easiest trinket to get hold of. Simply keep running Trial of the Champion until you have it. A very good trinket for the fresh 80 warlock.
Sundial of the Exiled
Not as good as the Abyssal Rune but about as easy to get hold of, this one requires no luck and nobody can out roll you on it. Forty emblems and it's yours.
Blood Pact is a weekly column detailing DoTs, demons and all the dastardly deeds done by warlocks. If you're curious about what's new with 'locks since the last patch, check out WoW.com's guide to patch 3.3 or find out what's upcoming in Cataclysm from the BlizzCon 2009: Class Discussion Panel.