Combat has become a secondary dps spec.   For single target dps, it comes behind Mutilate, especially Murderable mobs.   However, it does better in trash, multi-add fights, interrupted fights.    It is also the best source for Savage Combat (2% more physical damage), probably better than Arms Warrrior (Blood Frenzy) over Fury warriors nowadays.   So it is advisable to have at least 1 combat rogue in a raid.

Spec wise, it is not really the case where Sword spec dominates everything else.   The spec itself is still best, but talents like Prey on the Weak boost crit value which boosted Fist/Dagger spec.   Also, through itemization, swords only having 1.5 - 1.6 speeds to them compared to daggers 1.3 - 1.5 speeds gave Dagger offhands another boost.    It'll be better spec'ing around the weapons you can obtain, using a spreadsheet to check which is better, and not gunning for a particular weapon spec.

There are 2 schools of thought for combat specs, they are said to be very close, some debate on which is actually better.    They are the 7/51/13 build, and the 15/51/5 build.    Note, although it's a sword/CQC spec, you would move points to where your weapon spec lies.   If you're using 2 weapons of the same type, then the extra points are filler you can put anywhere.    For specs of different weapon type, the points you should sacrifice should be from Sword Spec, go 4/5 sword and 5/5 in the other for maximum benefit.

Now, I will go into each spec type with more depth.

7/51/13

This spec goes into Serrated Blades, and sacrifices a point in Imp SnD.   

Glyphs it would go for is Sinister Strike, Rupture, and Blade Flurry or Slice and Dice if you're having trouble with upkeep.

The goal of this build is to, while maintaining 100% SnD uptime, also try to maintain near 100% rupture uptimes.     Cycle would be 4+/5r, nothing else just SnD and Rupture.     Even with Energy Pooling, there will be times where Rupture would still be up, and can't override due to previous higher AP values, those times you'd just SS more til you can Rupture again.

15/51/5

This would be similar to the vanilla wow combat spec, going into Ruthlessness and Lethality, maxing SnD, but also sacrifice points in Sword Spec if applicable.

Glyphs it would go for is Sinister Strike, Slice and Dice, and Rupture

This build introduces a 3 finisher cycle, 3+s/5r/5e.    While maintaining 100% SnD uptimes, rupture when you can, and can probably squeeze in an Evis with decent luck on procs.

Poisons

For either, your choice of poisons will be Deadly Poison, and Wound Poison.   With the 50% proc rate, and lack of talents in Imp Poisons for combat, Wound Poison will out do Instant Poison, always.    Here's an AP crossover chart if you do happen to have points in it for whatever nub reason to make Instant Poison the better poison.

As for what poison to put on what hand.   Depends.    If your offhand is 1.4 speed or faster, put Wound Poison on it, Deadly on the other.   Else, put Wound Poison on the main hand, Deadly off.

Update for 3.0.9+:  So with 40% SnD now, people at the higher end of gear actually has Wound Poison surpassing Deadly Poison.   Not a whole lot, maybe 10ish dps.   But Dual Wound has become the ideal setup.    Provided your gear is good enough for it.    Exactly when?   Check a spreadsheet.