>Do any theoretical computer scientists seriously worry that a proof that P=NP would threaten cryptography?

Yeah, that's how crypto works as a field. As soon as there's a shadow of a doubt that something is secure in even the least likely edge case, it gets tossed out. MD5 was basically considered dead once someone showed to how create a collision with (correct me if I'm wrong) two very long, very unlikely, very similar (?) plaintexts. It didn't threaten anyone's actual use of MD5 in an immediate sense, but MD5 was shown not to be reliable and in the crypto world an algorithm's reliability is sort of treated as binary.

(I apologize to the true security folk in the crowd if I've oversimplified something here.)