https://github.com/bitcoin/bitcoin/pull/215
This pull is prompted by changes I'm making to ClearCoin, and should apply to any service where customers will owe the service bitcoins. It removes the account balance checks from the RPC move command.
I'll use it to create accounts associated with users that keep track of how many bitcoins they owe; for example, if I owed 1 bitcoin ClearCoin will tell bitcoin:
move 'gavinandresen' 'total_owed' 1.00
Assuming I'm not carrying a balance, that makes the gavinandresen account balance -1.00 BTC. When I pay to one of the addresses associated with the 'gavinandresen' account, the account balance will be automatically credited.
If I were a professional accountant I probably would have written 'move' this way to begin with...