I'm with Andrew on this one.  Obama emphatically and publicly stated he wouldn't run at Clinton's vice presidential candidate on March 10th, and I doubt Clinton would take a second seat either.  She'd prefer to stay in the Senate.

Stan, I understand the mountain of polling results showing that Democrats and Democratic leaders want the two to run together, but it would weaken the ticket.  Hillary Clinton's negatives are too high in those very same polls.  Obama is going to pick a V.P. that will maximize his appeal to key voting groups, like Hispanics, women, and workers.  Clinton would do the same.  She doesn't need any help with Blacks, and I'm not sure Obama's wildly supportive 18 to 30 year olds would be as enthusiastic with Clinton at the top of the ticket. They wouldn't switch to McCain, but they might not show up.  Clinton and Obama really are friends when the get offstage, but they're not going to get offstage for a long time.

Finally, the Kennedy-Johnson example doesn't apply here.  First, that was the only time in modern history that a vice presidential candidate brought in a big state.  It's very unlikely that could be repeated, and, besides, Obama can take New York by himself.  Second, Johnson's unhappiness as V.P. and his run-ins with Bobby Kennedy were legend -- enough alone to deter Clinton.  Third, that was a backdoor deal done back when this country and the Democratic Convention were far different places. 

It would be very difficult for Obama to sustain his "change" candidacy by stepping back in history to pick Hillary Clinton has his running mate.