First reaction: well, it's about time somebody gave that thoroughly undistinguished piece of ideological schlock the trashing it so richly deserves. Too bad they didn't make a better job of it. (As I understand it, "Piss Christ" was far from "destroyed" in the attack - just slightly damaged.)
Second reaction: oh, crap. The all-but-forgotten Andres Serrano has now been granted another fifteen minutes of fame. And the remains of "Piss Christ" will probably soar, in cash value, far beyond the undamaged original. (I mean, it's not like anybody cares what the thing actually looks like - this is conceptual, not visual "art" - and the attack way ups the conceptual ante, here.)
Third reaction: maybe the attackers can argue that their attack was, itself, a piece of performance art...
Hat tip to Paul, for e-mailing the article.