Looking at the various cinema sites mirrormask has vanished off the face of the earth near enough. Cineworld are showing it in Renfrew and the others are claiming its on at cinemas then denying it though whether thats because its after thrusday night closing time and they are on to a new schedule I don't know.
Its already out on DVD in hong kong and the US
IMDB claim a March 3rd general release date in the UK, although it seems none of the chains have picked it up. Whether this is because it is too weird or simply too bad, I dunno.