I think this is a pretty smart tactic by the Clinton camp. It smacks of compromise, which is something everybody in the Democratic party wants right now: an end to the infighting. And it also begins the murmuring of "Obama as Veep." This is kind of what happened with Edwards, I think, in the latter days of the '04 race. People liked Edwards, but there were some doubts about his experience. Everybody would have loved to have him as a veep, but he was running for President. Now Clinton gives the folks who like Obama but have doubts about his experience an easy out: support him, yes, but support him as running mate.
"What are you talking about?" ask the Clintonians. "She never said anything about who'd have the top spot!" No, she didn't, but traditionally, the VP is someone younger, someone with less experience who is being "groomed" for the Presidency. The only recent exception is Bush/Cheney. Kerry/Edwards had this relationship, as did Clinton/Gore, Dole/Kemp, Bush/Quayle, Reagan/Bush, Mondale/Ferraro, Carter/Mondale, etc. Since Hillary is 1) older, and 2) is perceived as having more experience, it's natural that most people would envision a Clinton/Obama ticket faster than an Obama/Clinton one (also partly because Clinton is seen as NEEDING Obama, and not vice-versa).
It's a clever ploy: pigeonhole him into a running for vice-president. Not sure what Obama can do to stop this. Of course, Hillary basically controls the rhetoric right now. Maybe Obama needs to drop that "superdelegate bomb" that TrailHead was promising...