I think it's a great idea.  Seems like an efficient formalization of options/futures rolled into the blockchain itself.  It should help to stabilize price and, as you said, encourage adoption.

EDIT: IIAP and think the contingent block mining would be easier to implement (and it's conceptually simpler).  However, as others have mentioned 'OpenTransactions' already seems to be approaching 'p2p contracts' and that may be objectively better if one is particularly attached to the current blockchain.  IMO, CBM is a bit more elegant (and it allows the miners to get directly involved in predictions and doesn't preclude the additional options/contracts markets from developing around it either).  There is something to be said for leaving a working system alone though and building onto it.

Have you considered what would happen if a significant number of miners enter the contingent market exclusively and guess wrong?  All that hashing power would be lost and the original network would be less secure as a result.  Or are there enough competing interests to balance this out?