IIRC, the last game they announced at E3 was the Diablo II expansion. They typically announce stuff either when it's ready, or at ECTS in September.
EDIT: And I wouldn't expect them to announce a new StarCraft game until they finish the current one (StarCraft: Ghost) since it's story is supposed to lead into future SC titles.
Sincerely,
Jeremy Dunn
This comment was edited on Dec 11, 20:49.