Well, it's a two-fer -- helps Obama but a) keeps her on the national stage and b) further rehabilitates her image with Dems/ prepares the ground for a later run if need be. (Hmm, that got up to three-fer or four-fer.)
That's what she did quite well at the Dem convention.
I think it'll be understated though, at least for now. Palin seems to be invoking Hillary in widely-seen appearances but then not (anymore, due to boos) in smaller, mostly Republican appearances. I expect she'll invoke Hillary in her convention speech, and I think that's one way to deploy her -- some sort of appearance (CNN would be good) where she says "hello, I AM Hillary Clinton, and I don't approve this message."
But I think the Dems are hanging back and letting Palin self-destruct for the most part, which is the correct approach IMO.