It is a possibility that the Diablo Series has come to a standstill after a part of the Diablo project staff quit from Blizzard and started their own company(or so I have heard). Pray hard that they do work together again to a common goal ---->
DIABLO III
Anyway, if that's going to happen, the next installment will most probably be out in like the next four to five years. In fact it may two to three years after the initial announcement of the commencement of Diablo III is made. For your information, Blizzard is still busy with the upcoming FPS ------>
STARCRAFT GHOST and their recent WoW Beta..... It may be a long time before any trace of Diablo III is ever going to go public or even at all.....