If Obama should serve two terms, I think it's highly unlikely Joe Biden will see all 8 years as Veep. It could be his health (yes, it's been a long time since his brain aneurism, but there was surprisingly no brain scan in his released medical records) or his propensity for gaffes, or just a strategic move. He could even be finished after one term. All this is just a hunch, but I'd give it 50-50 odds.
With Janet Napolitano's apparent selection as Sec. of Homeland Security, it puts her in a high-profile cabinet spot. If Biden were to leave at some point, watch Obama put Janet in as veep. Why would he do that, you ask? First, despite being a twice-elected, fairly popular governor in a red-leaning state, she is not an exciting personality or public speaker. She has also long been the subject (maybe unfairly?) of a whisper campaign about her sexual orientation (remind you of another Janet in a Dem cabinet position?). She would probably never be put on an elective ticket at the national level. But if she were APPOINTED as Vice-President, that could circumvent her electoral weaknesses and leave her running for President in 2016 as a sitting VP.
This would also accomplish several pluses for Obama. It would add to the historic nature of his presidency by capturing the first black AND first woman mantles. It would also essentially freeze out a move by Hillary in 2012 or 2016. And though it would not take the shine off of Sarah Palin as far as we conservatives go, it would definitely take away from the novelty of a possible Palin presidential run that might help appeal to moderate and independent women.