One thought that occurred to me listening to Ted Kennedy this afternoon: In addition to all the other reasons for his endorsement--the genuine excitement surrounding Obama, Clinton fatigue and Kennedy's frustration with the Clintons' hardball tactics, a sense that Obama is a natural heir to JFK--I wonder if race didn't also play a role, by which I mean the Kennedys' legacy on race. JFK didn't have a shameful record on race, but he wasn't exactly out in front on the civil rights issue either. (This site is a little elementary--I'm sure it'll make the historians out there cringe--but it gets at some of Kennedy's ambivalence. Also, don't forget that it was Bobby Kennedy who, as attorney general, authorized the FBI wiretap of Martin Luther King's phones.)
Obviously the family record on race since JFK's presidency has been pretty impressive. Still--or, perhaps, as a result--I can't help thinking Teddy might have wanted to get his family unambiguously on the right side of history once and for all, and that he saw a chance to do that by endorsing an incredibly promising African American candidate.