I don’t think she will do it.
She will stay Secretary of State for the balance of the first term. If BHO is re-elected, she will resign shortly into the second term and no one will blame her as the SecState job is brutal. This will allow her to publish one more book and do the money-raising chicken-dinner circuit. She will then run in 2016 to succeed BHO, taking credit for thegood that she did as SecState but distancing herself from BHO’s screwups.
If BHO would lose in 2012 , she would be even better positioned for ’16.
Also, dropping Biden in 2012 (unless he is sick) will be seen as intensely disloyal by the American voters. No President has changed VPs for an election since FDR put Truman on the ticket. Nixon ran through a couple VPs but that was because Agnew was even more crooked than Tricky Dick and even then it wasn’t for an election – it was mid-term and Agnew had to resign the position.
It would be near political suicide to switch VPs in the 21st century just based on politics. Even George W. Bush didn’t do it with Dick Cheney which would have been a good move for the party. Cheney had terrible numbers and was never a candidate for President in 2008. Putting a fresh face on the ticket would have allowed Bush to weasel out of some of his mistakes and would have setup a successor candidate (rather than the very limp and inadequate McCain). For that matter, GWB could have put McCain on the ticket and given him a better standing to run for President with fewer competitors in the primary.
If Biden should become very ill, Clinton is the obvious choice for VP. I have nothing against Mr. Biden, so I don’t want anything bad to happen to him, but if for some reason he is faced with a life-threatening illness, Hillary will be the logical and politically-prudent choice for VP.
Biden is here to stay if he is healthy. Look for Clinton on the ticket in 2016.