There won’t be any riots. Too much to lose...and not enough support in the black community. Maybe a few gangbangers getting frisky, but that is it.
The modern Left is built on a protest/riot core. They can't NOT use that to maximum effect now, because that's what they are, what they do, and there is no better day for it than November 5 2008.
* - Win: re-election in 4 years, won or lost, is a relatively non-controversial process; assuming an 8-year reign, his banner will not be rallied 'round so enthusiastically out of dull familiarity. Loss: McCain serves out 4 years, Palin follows for 8, the Left can't push a "minority" agenda against her.