I'm not so sure about that. Obama is not like the Jesse Jackson type candidates of yesteryear. Obama is a bona-fide candidate who has already been elected to high office. Nobody ever took Jesse Jackson's presidential runs seriously and even Jesse himself would show up at the convention to endorse the nominee.
If the Clintons end up stealing the nomination through parlor tricks (i.e. having Michigan and Florida delegates count after all and sewing up the super-delegates), I just don't see Obama toadying up to them and accepting a VP slot or otherwise endorsing them in any way. And I would expect a lot of Obama supporters to sit the election out. I don't see many of them voting Republican but I do see them sitting out in large numbers.
I also see many blacks walking out of the Dem Convention if Shrillary prevails and they count the MIch and Florida delegates the first time around...They won’t stand for that, especially when their man is clearly better than the Beach...