Barack Obama is saying he doesn't want the second spot on a Democratic ticket even as both Bill, and Hillary Clinton have begun to suggest this option regularly. I think this is because strategically the option works better for Clinton than Obama. First because having Obama on the ticket would gain Clinton more voters against McCain, where it's less likely that loyal Clinton voters will do anything other than vote for Obama if he's the Democrat to support when the time comes.
Also, many would argue Obama is the more likely "VP" candidate due to less experience - experience he'd then get and go on to be president.
Will this strategy work for Clinton? Probably not so much in the remaining primaries though it might soften the Obama momentum going into the convention, but the strategy is very likely to appeal to party insiders who ultimately may make this decision. They'll want a solution that alienates as few people as possibly, and this may be what they come up with.