Kevin Drum argues that the big tax/penalty/mandate debate is more than just semantics, given that John Roberts made a big deal of it in his decision. Perhaps. This is worth a quick once-through:
1. Substantively, what you call it makes no difference whatsoever. I think Ezra Klein nailed that one completely.
2. Politically, in terms of 2012 electoral politics, I'm going to insist that it makes no difference whatsoever what the thing "really" is. Republicans are free to call it whatever they like (as are Democrats). There's not apt to be any cost at all to anyone if, for example, they are radically inconsistent about it.
3. Legally, Drum has a point...but that's all in the past now; there's no reason that our substantive understanding of a policy has to be controlled by the legal understanding of it; and anyway, I strongly suspect that Roberts would have found some other hook if "tax power" wasn't available, although that last bit is obviously speculation.
So, yeah, it's really just semantics, and nothing at all is at stake in what anyone, Romney included, chooses to call it.