(As an aside, I've wondered what Blizzard would do if D&D ever really started having an online presence ... that would seem to be a strong motivation for Blizzard to simply squash WotC out of the online gaming market.)
Ignore them, like it does with the other MMO companies?
I can't imagine Blizzard ever feeling remotely threatened by WoTC. They've probably made more off WoW since its release than the last ten years of D&D.
With regards to the actual release of the Tabletop.... Well, after Chinese Democracy managed to actually get released (albeit after 14 years...) I guess anything's possible.