I know at least some people are using gpg to sign contracts before making trades.

There are still a few exploits someone could do even with a signed message.  They could modify the system clock to post date a siganture, rendering it invalid.  They could revoke the key, which means that any previous signature could be blamed on a malicious user.  There are a few other attacks as well.

I was thinking of a service that would notarize a signed message by signing with a neutral third party's key.  This indicates that the neutral third party believed the signature was valid at the time it was posted, limiting the possibilities of someone calling foul play.  It would also archive the message, with notarized signature on the website, either in clear-text for the world to see, or encrypted to the gpg keys of the parties involved, so either party could extract the contents later.

Charge would be very nominal.  Probably 0.01 btc to start.

My questions are: Are there a lot of people who actually sign gpg statements before issuing transactions?  And if you're one of these people, would you be interested in such a service?  And if so, is 0.01 btc too cheap? ;-)