Very good. Allows use of multisigs without installing bitcoin-qt and downloading the blockchain.
However you shouldn't use the word "escrow" to describe this. This isn't escrow because the escrow agent cannot run off with the money. Also "escrow" agents are heavily regulated in many parts of the world, while arbitrators, adjudicators, mediators, judges, reconcilers, referees, umpires, etc are not subject to that regulation.
Changing the word doesn't change much in law, but its bad to give the wrong idea to lawyers since multisig transactions are not something that's ever existed before to my knowledge.
Also you might want to look at
http://www.bitrated.com/ which does the same thing but requires an account and has some other features. I don't think it can be easily used with an online auction site, like bitmit.net with multisigs. Your site seems much better for that.
edit: some other thoughts.
1. If the public key is copied in with a mistake it could lead to loss of funds. Perhaps we need a standard for base58check public keys? According to the wiki (
https://en.bitcoin.it/wiki/Base58Check_encoding#Version_bytes) you'd use the version byte 42 but it's marked "proposed" and I can't find any other info on it.