Why cracked encryption is a good thing

Some might argue that it has been a bad year for encryption. After all, just as the last decade was ending came reports that the algorithm that is used to encrypt GSM mobile phone calls (as used by some 4 billion people around the world) had been cracked wide open. Now this has been followed by the announcement that 768-bit RSA encryption has been cracked. I'm inclined to think that this is a good thing, and am happy to explain why...