The same thing happened to me a few days ago. I just went into the WoW folder and ran WoW.exe instead of the launcher. The next time I ran the launcher, the problem went away and WoW opened up normally.
Edit- just had it happen again. I closed the launcher and re-ran it, problem went away and WoW was accessible.
Edited, Jul 10th 2011 11:09am by Matheou