Ok, I have seen the statements made where you have to have your fastest weapon in your main hand in order to get faster skill raises. This is nothing but a myth. The same goes for DPS damage. Again a myth. Let's take the following example into consideration, along with a little math.
Let's say that your main hand is a positive attribute, while your offhand is a negative attribute. Now, let's get a fast and a slower weapon.
Battle axe with a delay of 32, and a longsword with a delay of 28. Some people on here seem to think that the longsword will go in your main hand since it's the faster of the two. This is a commonly made mistake when dual wielding. Now then, a little math:
The two weapons have a difference of 4 points between the two of them. As stated above, if you place the axe in the main hand and the longsword in the off hand, that result is a +4 chance for dual wield skillups. Now then, if you reverse the pair, then the result becomes a -4 resulting in a decreased chance for the skillups. You may be hitting more often with the longsword in the main hand, but your not doing more damage. Meanwhile, the better of the two weapons is sitting there and waiting for a chance to hit. With that -4 offset, it doesn't allow for to many hits to happen. Now, with the weapons placed with your axe in the main slot, and the sword in the secondary, that becomes a +4, resulting in a much higher chance for skillups, damage, and if your weapon has it, procs.
The end result? Your better off putting your faster weapon in your offhand than in your main hand. Wether your DPSing for a group or running solo, you will find that this works more to your advantage than your detriment.