I thought it's consequent to use Bitcoin address signing for this contract
Yes, but you have to use Bitcoin itself to sign it. No one agreed on the signature algo, so everyone else is doing it wrong and they need to switch to use Bitcoin's. If you would have signed it with bitcoin-qt or bitcoind, this wouldn't have been a problem.
Thanks for clarification.
If that's the case (everyone doing his own thing), it is not useful yet, since people not working with bitcoind (like /me) can't reliably sign / verify.
I believe everyone is moving towards using the bitcoin-qt system for signing, but I'm not sure. I'm signing with bitcoin-qt, let me know if that is insufficient. I don't have a PGP key yet, and this signature is easily associatable with the coins in the account it is coming from.
Contract signed from address: 12mwAvJywtQF5rJXHzdBQd4yq8EVD7NZqX
Signature:
HyhgIo5QfmPpuQw2iPAakBhp0sCP3BnnBjXiojs+rsaCcs4QGoo74xDUOthtn3wlKTmKDA7qYhZUR9Qhi3K/Glc=
(signed with bitcoin-qt 0.7.2-beta)
Once John has signed, I'll initiate the transfer.