Well... it's hard to say. Mariah Carey fell from grace quickly but came back with a vegence. Drew Barrymore is another person who was pretty messed up for a while then came back to respectable places. However, the things that might get in Britney's way are:
1) Whether or not she can sing. I'm not convinced that she can. Mariah's got a hell of a voice. I'm going to guess Britney can carry a tune, but is it any better than Mariah?
2) Her writing talent. Again, Mariah can write. Britney hasn't proven to me that she can write.
3)How forgiving people are of a long-term downward spiral. She's been on this path for a while. Mariah's was more short term.
4) The children. The fact she's this crazy and has two children isn't going to bode well with people.
She's got a lot working against her, but the important thing is how her people spin it and that she shows redemption. Everyone loves a redemption story. All she needs to do is go on some talkshows (thinking Oprah), articulate what went wrong (I have a sneaking suspicion she can have a legitimate sob story), cry, and show she has changed and is better. If she can get on the good graces of some powerful people, she can at least not be a joke.