Yep... there are many checks I left out of the verify method 
1) I need to check that nonce < MAX
2) check that a != b
1) I need to check that nonce < MAX
2) check that a != b
Aye, you've got me wishing I'd just kept my mouth shut and waited to see if it was in the live code, could have mined a bounty out if not
Regardless, it's a good PoW system and will be considering using it, or a variation of it with a customized salsa20 in the future.