Here's our Combat rogue mastery ability as it exists in the current Cataclysm beta:

Rogue

• Combat Your main hand attacks have a 10% chance to grant you an extra off hand attack. Chance increased further by mastery rating.

Judging by this ability alone, a Combat rogue would logically want to equip a fast main hand (MH) weapon, and a slow offhand (OH) weapon. This is completely backwards to what we're used to.

First of all, we're currently using a slow main hand weapon because many of our abilities are based on a percentage of our main hand weapon's damage. For example, Sinister Strike is our main combo point builder, and the slower our MH weapon is, the harder it will hit. (Our two main finishers, Eviscerate and Rupture, are not based on weapon damage, so wouldn't affect your decision on weapon speed.)

As for our offhand weapon, the primary reason we've wanted a fast one is because of Combat Potency. A faster weapon means more energy, which means more attacks. I'm suspecting that an OH attack proc from the mastery bonus will also qualify for a Combat Potency proc, though.

So, do we equip a fast MH and slow OH for our Mastery bonus, or do we keep things as they are? Or, should we balance and run with two fasts or two slows? I think we're gonna need a spreadsheet here. Another thing to consider is the new Shiv (Now removes an Enrage without Anesthetic Poison! Yay! Perhaps I should macro a Thistle Tea to the Shiv to essentially reduce the cost when I need to remove the enrage?).

As a side note, this doesn't really affect our poison choices. Assuming that level 85 Instant and Deadly poisons are equivalent to their level 80 counterparts, we'll just have Deadly on our fastest weapon, and Instant on our slowest as we do now. If we're using two weapons of the same speed, then Deadly would likely be on the MH, because it hits more due to our special attacks.