Noam, I agree with you about the silliness of calling Massachusetts an "upset." At least, as it applies in the rational world.
But in the madhouse of political media, I think such talk reflects a problem for Obama. His expectations--especially within elite media--probably got too high. It may not be fair to say Hillary was the underdog tonight, but I certainly heard talk in the past day or so that she might be headed for disaster.
That said, if the delegates split roughly even and Obama still wins a bunch of states, his bulging bank account sends him into favorable upcoming contests (i.e. the MD/VA/DC "Chesapeake primary" next week) in pretty good shape.