Here's what I know (thanks to candideyes for the information). The only offer from Universal is a direct to DVD release (no doubt remembering the utter disappointing 'Serenity'). The theatrical version would be about 10 years away with an entirely new cast.
Lucy had alwasy been pretty determined to do a theatrical version, but is now warming up to the idea of doing a straight to DVD release.
Both Renee and Lucy are interested if the story is good. But that's a big "if."
Rob is not sure if it will be post-finale, if it will be an uber, if it will be a movie set during the series, a movie set before the series (which means no Gabrielle, or very little Gabrielle).
So as of January 16, 2007. The only offer is a direct to DVD release and there is no story for the movie, either.