I'm just speculating, but I don't think she's serious about fighting for the 2008 nomination any more. I think she knows she'd lose, and that mainstream Democrats would resent her dividing the convention nominees.
What I think she wants is to force Obama to acknowledge her power within the party and to let her choose his running mate (someone who'd look good but wouldn't help him win, since her future presidential campaign depends on his losing the 2008 election. I think she's setting the stage for 2012.