Sadly, IGN's recent interview with Paul Sams doesn't leave me with much hope for another Diablo game in the next year. As he said, we'll get one eventually, but he sure didn't make it sound like a Diablo game is anywhere close to imminent. Maybe in 2009?
I guess that's ok, though, since Hellgate: London is coming out soon, and there seem to be a lot of other decent games coming out between this fall and next. Maybe Blizzard will surprise me just as those are wearing off.