The cheat is pretty trivial - when you get a "share", before you turn it in, see if it matches the current difficulty.  If so, claim the 50BTC for yourself.  Otherwise, hand it in for the share.

Let suckers pay for your solo-mining.

I don't see anything in the protocols that would prevent this, and without extensive auditing of every work unit handed out I don't see how the pools could even detect it.