Yesterday, at the BFI screening of The Tomb of the Cybermen, Steven Moffat revealed that Matt Smith will be back to work on Doctor Who after his filming stint on How To Catch A Monster. During his introduction of the classic Patrick Troughton story, the current showrunner stated that Matt is "not going" (not leaving the show, that is) and will "be back" after shooting the Ryan Gosling-directed movie.
Smith resumes filming on Doctor Who in April for the 50th Anniversary special, then shoots How To Catch A Monster in May and then returns to the UK to film the 2013 Christmas Special.