I haven't read any evidence of what happens to wort when you freeze it, but I would imagine that when it thaws, the composition should still be relatively the same, no?
I have heard of people pitching their yeast, leaving their wort in a cool area to ferment a lager, and having THAT accidentally freeze. They've warmed it up and got it going again (I believe it happened to John Palmer; he mentions it in "How to Brew"), so your situation shouldn't be any worse than that.
Either you dump it and waste it, or pitch the yeast and it turns out awful and you dump that. Might as well go for it once the temps get up to where you want to pitch.