I'm pretty sure it's been passed from director to director over time. I know there were some rumors about Tim Burton directing it, with Johnny Depp playing Sweeney, but I don't know the validity of it.
On IMDB, it says that there's a Sweeney movie slotted for 2008, directed by Tim Burton, and that it's in pre-production, but it has no info about a cast or anything. Apparently Sam Mendes was originally set to direct it. *shrug*
It'd be interesting to see what they'd do with it. I'd like to see what Tim could do for it, but I'm not sure I'm so keen on having Johnny Depp play Sweeney. I love Johnny, but I'm not sure how he'd do in the part.
I'm pulling for Christopher Walken, myself. I know he's had a theatre background, and can sing...I think it'd be interesting to see his rendition of Sweeney.
"Cages or wings, which do you prefer?"-- Tick, Tick...BOOM!