I think that when payouts are cut and mining power decreases, the difficulty will reset to preserve the 6 blocks an hour. Also, transaction fees will be an incentive to keep mining as payouts decrease.
It will, but at the least it will be a rough 3-4 weeks until it does, and the question is why do we want that?
I'm not an expert on the Open Transactions platform mentioned by Stephen above, but I believe that it doesn't require a trusted third party (e.g. VISA, AMEX etc.). It seems it could allow instant payments so would be complementary to bitcoin.
I think it was developed by someone known as Fellow Traveler. I just listened to this Cypherpunkd episode today
http://agoristradio.com/?p=234 about it. I'm interested in learning more about it.
My understanding is that Open Transactions rely on an external server for verification, so in that sense they are dependent on a third party. If the server's down, the transaction can't be marked spent. The "Open" part about them is that anybody can set up a server and issue currency.