For what it's worth, I actually buy the Clinton stance on immigrant driver's licenses, though I think Mark Penn (via Ambinder) articulated it better than she did during the debate. All she meant was that, in the absence immigration reform on the federal level, individual governors may have to resort to some unorthodox measures, which is what Eliot Spitzer did by contemplating driver's licences for illegal aliens. (Whether we like it or not, illegal aliens drive--and get into accidents--like the rest of us.) Given that she's a U.S. senator and that she's running for president--both federal offices--and that she's already said where she comes down on the federal aspect of the issue, I don't see why she has to take a rock-solid position in a state-level debate.
That said, it certainly sounded like doubletalk when she discussed it last night, thanks to her mangled delivery. And if I were John Edwards and Barack Obama, I would have jumped on it, too. But the substance of what she said didn't seem problematic to me.
Update: A commenter points out that her response wasn't really substantive. Agreed. I should have written, "But the crux of what she said didn't seem problematic to me."
--Noam Scheiber