Arbitration clauses are a common feature of commercial contracts*. As Bitcoin becomes a more common way of transacting business I can imagine there being two-of-three multisig used with an arbitration company.
How the scheme would work is that one key is held by the buyer, one by the seller and the third by a mutually agreed upon arbitration agent. The agent would do whatever is ordered by the arbitration that is triggered by a dispute between the parties. This would allow for arbitration orders to be both binding in law and in practice (becomes the bitcoins would be sent immediately).
After writing this note I saw that there's a similar idea expressed in the "Contracts" section of Bitcoin.org.
*I'm not a big fan of arbitration clauses for low value contracts. I think lawyers tend to just stick them in by default without considering how that could ultimately impact their client. Given how expensive arbitration is in Toronto, many people would be better served by small claims court (where taxpayers pay for the judge and room rental).