It makes sense to me that a mainhand that is slightly slower than your offhand would be detrimental to your dps. This is because your offhand will always be hitting slightly before your mainhand, which gives a good chance that your offhand will be eating significantly more windfury procs.
How does this effect two weapons with the same speed? If both of your weapons are always hitting at the same time, does the game have an order it chooses when looking for possible windfury procs, or is it just random whether it looks at your mainhand or offhand first with equal weapon speeds? I am saying this because I have two 2.6 speed weapons, and it does seem like I get more offhand procs than mainhand procs... could just be imagining it though I guess.
On a side note, check out this screeny:
http://img187.imageshack.us/img187/5...erhastedl9.gif
Those weapon speeds were originally 2.6. This is with almost every possible haste buff proc'ed: Dragonspine Trophy proc, haste Meta Gem proc, haste potion, bloodlust, flurry, and a mongoose proc. I suppose it could go faster with the crafted mace proc, another mongoose proc, abacus, and maybe some haste gear.