It looks like there'll be an ascii-plaintext challenge (we won't know the full plaintext - just that it's ascii, and long enough to be unambigious)
Ick. Why overly complexify things? A known plaintext attack would be far more straightforward. After all, the goal is to recover the key, not the message.
I think a completely known-plaintext attack would not impress the masses. Consider how often crypto illiterate programmers implement ciphers (such as Vigenere variants) which are obviously vulnerable to known-plaintext attacks. The idea seems to be that if you know the plaintext, what do you need the key for? _We_ may know better, but I think we are in the minority. For a slight increase in the computational requirements, we could end up with a break that the "DES is good enough" people would have a _much_ harder time downplaying.