If you're like me, most of your specs in WotLK have involved at least 41 points into Assassination at all times. With HfB Mutilate being competitive in end-game PvE, and Mutilate/Prep being our PvP spec of choice, my daggers have been getting a ton of use. I swap in my Librarian's Paper Cutters for PvE and my 1.8 speeds for arena, and that's essentially the only gear change I have to make. There's nothing wrong with loving Mutilate, even after the nerf it has been a solid performer for Rogues in all environments.
The beauty of Mutilate is that the playstyle is similar in both PvE and PvP. Changing the filler points around can turn a build from arena-ready to eagerly-awaiting-Ulduar. This synergy between both major environments allows you to truly master Mutilate's nuances and best practices. If you're attached to this double-dagger attack, then here's the two specs I'm recommending for 3.1:
PvE : HfB Mutilate, standard
PvP : Muti/Prep, Blood Spatter and Serrated Blades over Improved Eviscerate as resilience increases
Raider who casually PvPs:
HaT builds currently top the PvE charts on nearly every boss, and I suspect this trend will only continue as your party mates' gear improves and they are able to generate more CPs for your Eviscerate spamming finger. I find that this spec is also very flexible in that it can allow you to mix up your finishers as your raid may need. I have never raided with a Warrior in a 10 man instance yet, and so it falls to me to keep Expose Armor up. With the constant influx of combo points you'll see with HaT, keeping EA up becomes trivial; EA is one of the highest valued debuffs for physical damage dealers.
Shadow Dance is also an incredibly fun PvP spec that really lends itself to both arena and BG play. In an arena environment, it can allow for incredible flexibility (notice again how Subtlety is the flexible tree) while putting out significant burst with an Ambush Barrage (which I will call ABar from now on). You're likely to face a situation where you're outnumbered in a battleground, and the ability to teleport around while using openers on whoever you wish can provide a significant opportunity for taking out a 1vN situation. The amazing mobility of Subtlety also makes it the true Hunter-killer spec. Here's the two specs that fit best with the Subtlety mindset:
PvE : HaT PvE, standard
PvP : Shadow Dance, daggers / Backstab
With the buff to Slice and Dice, the nerf to Mutilate, and the fact that HaT has a maximum scaling point (5 CP / second) for its main CP generator, Combat is poised to reclaim some market share in 3.1. Eyonix has also indicated that further buffs are in store for our oft-neglected center talent tree, which will bring it back in line with HfB and HaT for PvE. Rogues will either rejoice opportunity to return to our main raiding build in vanilla and TBC, or cringe at the thought of binding a key to Sinister Strike again.
While I currently feel that Combat PvP is too weak to perform in arena play, it does tone down how badly we are destroyed by our plate-wearing counter classes. Adding frontal avoidance along with significant toe-to-toe damage capabilities allow us to play with the Death Knights and Paladins in their space.
The issue is burst damage, as playing Combat means you will be best suited playing with a healer. In order to get your burst damage to place where you can realistically kill a target, you must sacrifice Preparation. This in turn leaves you open to being destroyed by any class capable of kiting and greatly reduces your ability to avoid damage. Once we have the 3.1 patch notes, I'll be able to see if Combat is truly PvP ready. Here are the builds I'd recommend (based off what we know about the upcoming mace buffs):
PvE : Combat PvE, standard (and currently competitive if geared)
PvP : Combat PvP, no prep
Mixing it up:
These of course are only ideas for what 3.1 will bring, and options for how to use your dual spec option. A PvE-only player may want to have HaT and HfB both available to them, based on the raid type or even a specific fight. A PvP enthusiast may vary filler points in their build to be able to flex their strengths and weaknesses based on their environment. Hardcore HaT raiders will also enjoy being able to revert to a better spec for farming without having to solo as HaT (which is the worst solo spec). It really depends on what you spend your respecs on normally, because that's where you'll see yourself using the dual spec option.
Outside of Combat's upcoming revamp, I don't see there being much on the horizon for Rogues outside of small balancing issues and minor adjustments. Blizzard seems to be content with where we're at, and not afraid to nerf us when they feel we're out of line. At this point in WotLK's evolution, I would start trying out some of the builds listed above, and find your happy medium with PvE and PvP builds. While Arcane Mages saw new rotations created with the Arcane Blast redesign, Rogues will be functioning nearly the same as always after 3.1. I await the rest of the upcoming changes with caution, as it seems that nerfs were left out of the 3.1 preview posts. Here's to hoping Rogues thrive in Ulduar and beyond.