1.4 Rogue Basics
Melee Attack Normalization: (affected moves: Ambush, Backstab and Sinister Strike)
Since patch 1.8 Attack Speed is no longer multiplied by your weapons exact weapon speed, but is now multiplied by a fixed speed based on weapon type. For Daggers it is 1.7 and for Swords/Maces it is 2.4. In short, this is a slight nerf to slow weapons and a slight buff for fast weapons. When choosing what weapon to use in your main hand, weapon speed plays no part in your choice anymore, but generally speaking the weapon with the highest damage range will tend to be a slow weapon anyways.
I'll start with a disclaimer. All the below comments are given without regard to stats, special attacks or procs. The reason being is that these seriously complicate the issue of which weapon is better than the next. Also, there is a lot of room for preference. Lets just keep it simple for now.
Daggers are required for Backstab and Ambush and in order to get the most damage from these moves, you should use the highest median damage range possible.
Swords/Maces are more effective if using a combat build. Since these weapons do not have an option to Backstab or Ambush your main attack will be Sinister Strike. The same rule applies for these weapons as for daggers. The best way to maximize you Sinister Strike is to Main hand the weapon with the highest median damage range possible. Since Attack power is multiplied by a static speed based on weapon type, choosing a sword/mace with a speed of 2.4 is better for instant attacks than a 1.7 dagger.
Main Hand: (in order of priority)
1) High Damage Range
Off-Hand, regardless of weapon choice, you will want the fastest weapon you can find but still with an acceptable DPS as it will excel mostly at using poisons for both PvE and PvP. In the Burning Crusade expansion it will be even more important to weild a fast off-hand as they will introduce an ability called "Shiv" which requires 40 energy but may require more energy if the weapon speed in that hand is slow. Your off-hand weapon will never do it's full DPS because it's damage is cut in half. (Even with talents its still reduced by 25%) Also, its important to know that your off-hand is not even considered in any of rogues special abilities. It basically just keeps swinging at a constant rate while fighting melee. The only special abilities that help an off-hand are ones that increase your attack rate, like Blade Flurry or Slice and Dice.
1) Weapon Speed - FAST (Especially after the expansion)