Something really serious and important (to Obama, at least) is up - I can’t imagine him taking time off for any other reason at this point. The birth certificate issue, as someone else mentioned, would be better taken care of by someone else rather than having to stop the campaign for two days to do it personally, and even a medical issue could probably be discreetly taken care of somewhere on the continental US.
Judging from the fact that there’s nothing else that I can think of that either can’t be done somewhere in the US, or can’t be done by someone who’s not Obama, I’d say whatever’s up does involve his grandmother. Maybe it’s something political, or maybe his grandmother is just sick with something terminal (ergo the being sent home from the hospital) and he wants to visit her one last time just in case she dies before the election. Either way, we’ll probably find out by the end of the week.
if she does die it would fulfill his COLD stereotype he’s starting to earn...how cold would that be? He doesn’t even buy his brother a “Hut sweet Hut” sign....doesn’t bail him out of poverty....for his Grandmother to die this close to the election without a visit by Barack would be politically “incorrect”.
This could be conveniently and credibly arranged while he is there anyway "to visit her." Very plausible ...