Id say sometime after they release the WOW expansion. Burning crusifix or something like that.
Thats to come out before the holiday season [wouldnt want to offend]. I wont speculate on game shows. But, it will be announced before this years E3, probably be a beta for E3 and we will see it for next years holiday season.
Anything later than that...well...Id just hate to think of anything longer than that.
Though its unannounced, ask yourselves...had Blizz been working on it...and for how long. They mostlikely been working on D3 for over a year. The origianal team started right away on it. So, I doubt its collected any dust.
Main question is ...will it be lame. Will it have the M rating, and will it live up to our expectations. I have read some lonng lists too. So I dont know. Hopefully they will keep a good story, 2.5 D and a few more characters. Oh yea, the M rating is manditory. I wont even look at it otherwise.