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.
One problem with using the existing block chain is that it ties up BTC in escrow. Suppose we want to wager 1 BTC on bitcoin difficulty 1 year in the future. To do this, we would need to hold 1 BTC in escrow for the next year. If on the other hand we loaned out the BTC at a hypothetical risk-free interest rate of 1%, we could earn around 0.01 BTC in interest during this year. Thus, there is something like a 1% annual tax on speculative transactions that use regular bitcoin.
On the other hand, if we a mine a bond that matures in one year, we don't need to forgo any interest to place bets on difficulty when it matures.
A intermediate solution, perhaps the best, is to allow the mining of bonds and then handle the rest using something like open transactions.
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?
I also worry that the system could become unstable (currency generation could balloon or collapse) if everyone made bad guesses. An arrangement that doesn't allow this to happen is described here:
2) keeping money supply growth constant is not difficult. One simple option is as follows:
a) issue 1/3 vanilla bitcoin [2 vanilla bitcoin bond blocks per hour]
b) mine 1/3 as bitcoin bonds which become vanilla bitcoin on June 1st of each year. [2 June 1st bitcoin bond blocks per hour]
(allow these bonds to be broken up into several types of mutually exclusive contingent claims)
c) mine 1/3 as bitcoin bonds which become vanilla bitcoin on Jan 1st of each year. [2 Jan 1st bitcoin bond blocks per hour]
(these can also be broken up into contingent claims)
Growth of the aggregate money supply is the same as before: 6 blocks per hour. it is just divided among more types of monetary instruments.
Basically it involves having three difficulty levels, first mining the bonds and then allowing the miners (or anyone else who buys them ) to break them up into contingent claims.
Open Transactions could handle splitting the bonds into contingent claims.