Multiple sources are reporting that anonymous parties in the know at Fox have decided not to renew 24 for a ninth day and that an official announcement could come any day now. Given the comments from NBC's Angela Bromstad last week indicating that NBC had already been talking about whether to pick up 24 should Fox pass, it doesn't come as a big surprise that Fox is tipping its hand.
Increasingly high production costs and diminishing ratings are cited as reasons the show will end its run on Fox. Fox's studio will surely try to shop 24 and a distressed NBC is likely the only network that would give it any serious consideration. Ultimately, I can't see NBC wanting to pay a high price for a show long in tooth that already isn't getting great ratings. But you never know.
If not, there's always the prospect of movies.