Satoshi created the transaction scripting system exactly for purposes like this.

"The design supports a tremendous variety of possible transaction types that I designed years ago.  Escrow transactions, bonded contracts, third party arbitration, multi-party signature, etc.  If Bitcoin catches on in a big way, these are things we'll want to explore in the future, but they all had to be designed at the beginning to make sure they would be possible later." -Satoshi

http://bitcointalk.org/index.php?topic=195.msg1611#msg1611

He said he was working on it for two-party escrows, but now that he's gone it might be something for the current developers to consider prioritizing.

Being able to so directly manipulate currency at the programmatic level allows for a full realization of the promises of smart contracts.  Contract enforcement through code without violence.  Decentralized, voluntary arbitration.  Realized Cryptoanarchy.  Imagine how much easier it would be to conduct business in such an environment.  It's not mentioned often here, but Bitcoin, in addition to dramatically altering the monetary system, really could revolutionize the entire system of commerce law.