This is the other thread that shows how haste effects can push weapons into the cooldown. This has the potential to make small haste gains net DPS losses (if they're at the edge of the 3-second delay breakpoint). Tseric seems to be making an effort to understand it and to bring the point to the developers.
http://blue.cardplace.com/newcache/us/79821827.htm
The other forum threads I've read are saying that WF5/WF5 does 7%-12% less overall damage (SS+autoattack) than WF5/WF4.
My own take on the theorycraft is that the shared cooldown hurts fast offhanders more than slow offhanders: if your offhand is fast, it will proc more than your MH (doing less damage than your MH) and suppress more MH procs. OTOH, a slow offhand will suppress fewer MH procs.
It seems that poorly-geared shaman will probably go with WF5/FT. Above some level of AP, WF5/WF5 will be the best despite the shared cooldown.
To the poster above me: RB is the worst OH chant possible. It is subject to the 50% offhand damage penalty, and is mitigated by armor.