The second title in the planned trilogy will be released in Japan on November 29th. If the original, Professor Layton and the Mysterious Village, does well enough when released in America later this year, the sequel would seemingly follow suit and be released some time in early 2008.
"Devil's Box seems to be improved in most every way over its predecessor, while retaining the same visual style and puzzle-based gameplay mechanics. The game uses a cartridge two times the size of the original, with room now allotted for improved animation and story sequences."
"Most of the puzzles in Devil's Box are new, Level 5 president Akihiro Hino confirmed at today's press conference. In fact, all the puzzles that are part of the main story are new.
All these details and a release date! Professor Layton and the Devil's Box will be released to Japan on 11/29. We're still waiting on Nintendo's release of the first one, now set for December 3."
quotes taken from IGN: