Eh, it's all speculation. But keep in mind the last two consecutive presidents to win election to two terms, before Clinton-Bush, was Madison-Monroe. So the fact that Bush I was the last one-termer doesn't really say anything about Obama's chances.
I was trying to think of the last time a president lost in a good economy, and decided it was probably 1968. Though the recession in 1992 was not that big, as well. But of course, we don't even know 2012 will be a good economy. Don't forget Carter, who won in a bad economy, and then lost in a still-bad economy. And if the CBO is to be believed, the economy might still be recovering in 2012:
http://www.cbo.gov/ftpdocs/110xx/doc11014/01-28-Testimony_Senate.pdf
CBO: Unemployment Rate Not Likely To Dip Below 9% Until 2012
That makes no sense at all. Republicans will want to beat Obama no matter the odds. And even if there are small odds, they won't be microscopic. And Romney's problems have nothing to do with his odds in the general, plus Palin has problems that also apply to the primary as well as the general, as I've already said.