There was already a rumor that even before the Biden selection Hillary turned down Obama. At that time, there was already a buzz on why Obama was not leading more in the polls. I am sure that Hillary calculated that Obama could lose 08 thus allowing her to run in 12.
Had she joined him, and Obama/Clinton wins, she could not run till 16. Undoubtedly she recognizes how difficult it would be for a VP of 8 years to run for PotUS.
If the first term is a disaster, then she would have almost no chance in 12 and would have to try again in 16. It would be prohibitively difficult to win in 16 after losing in 12 as VP.
Now if Obama/Biden wins she would actually have a better chance of winning in 16 from outside the administration. And an outside chance even in 12.
This calculus is even more obvious now with Palin.
I would say no chance in hell would Hillary join Obama now.