Written by Gaiman, directed by Dave McKean (an artist who's worked on the Sandman comics), and produced by Henson pictures, Mirrormask is an attempt to recapture the magic (in $ and viewer delight) of Labyrinth and The Dark Crystal:
MirrorMask centers on Helena, a 15 year old girl in a family of circus entertainers, who often wishes she could run off and join real life. After a fight with her parents about her future plans, her mother falls quite ill and Helena is convinced that it is all her fault. On the eve of her mother's major surgery, she dreams that she is in a strange world with two opposing queens, bizarre creatures, and masked inhabitants. All is not well in this new world - the white queen has fallen ill and can only be restored by the MirrorMask, and it's up to Helena to find it. But as her adventures continue, she begins to wonder whether she's in a dream, or something far more sinister.
This looks really really cool! I think Gaiman's work creates a rich visual experience and the fact that the characters can sometimes be, well, not quite so human means the Henson aspect could be really cool.
BTW, did I mention that Neverwhere, the BBC series that led to the novel, is available on DVD? (hint hint for birthday buyers out there!)