it took about 5 months from the release of the starcraft 2 beta until the release of the game.
so if the diablo 3 beta comes out soon, then we'll be looking at a january-march release date?
of course, this game might have a shorter/easier beta, since it doesnt need competitive balancing, and bnet 2.0 already is working (cause thats what starcraft 2 is on right?)
still, my money is on late january-late march.