Two daggers. You need them for Mutilate and Assassin's Resolve from level 10. Go get your Scarlet Kris heirlooms.
Not daggers. Some abilities reference one strike (Sinister Strike, Revealing Strike, Killing Spree, Mastery). That strike should be big. Dagger's one strike is small. Other abilities are normalized so speed doesn't help (Blade Flurry, Combat Potentcy). Use Thrash Blade or Mass of McGowen heirlooms.
Daggers allow Backstabs and bigger Ambushes. However you might not be using Backstab, Hemorrhage gives faster combo point generation. Faster combo points means more finishers. I find that in current dungeon situations, fights are very fast and backs available - Dagger shines here. While solo'ing, backs are not as available, or against dungeon bosses, fights are longer and finishers matter more.
Level 63 subtlety rogue vs level 60-63 Zangarmarsh wildlife:
Ambush hit: 1553-1799 crit: 3281
Hemorrhage hit: 571-711 crit: 1153-1447
Ambush hit: 1955-2233 crit: 4140
Hemorrhage hit: 571-731 crit: 1171-1205
Backstab hit: 1175-1341 crit: 2508
Knowing this, I would choose Dagger for Subtlety. Hemorrhage does the same damage either way. Ambush does 25% more damage with dagger. If you aren't ambushing and backstabbing, weapon choice doesn't matter.