Once 3.1 went live I found I could not launch wow.exe. My launcher was fine then hit play and nothing.
I then found out it was iolo technologies, and their products that was conflicting with WoW. For me it was System Mechanic, not for the firewall/virus scan but for the rest of it that I would prefer to not uninstall.
Blizz recommends "Right click on Wow.exe in your Wow folder->Properties->Compatibility tab->Input Settings->Turn off advanced text services for this program," but that did not fix it for me.
After a night of no wow, and stubbornly refusing to un install or to turn off the processes, I found my answer http://forums.worldofwarcraft.com/thread.html?topicId=16409621822&sid=1.
Look at the 16th post, from Ressie, basically changing the ioloHL.dll to _ioloHL.dll
Finally I can play my WoW. Hopefully no one else had this issue, but throwing out my solution in case it helps.