From what I've read so far on various forums, the optimal weapon selection in wotlk will be Slow MH+WF/Fast OH+FT.
From the looks of this, Lava Lash damage will be minimal. This made me wonder, wouldn't the skill work better if it was based on your OH's DPS (like Hammer of the Righteous for pally)?
If I have been mislead somewhere about Slow/Fast being better overall, please feel free to say so.