Let's say the FSF's bitcoin private key has just been compromised.
Current funds available at this address will probably be stolen, but that's not the problem discussed here.

Of course FSF will change wallet and public address... but donations to the old public address will keep on.
(Some people have stored the old address in their Address Book, some others will find the old FSF address on bitcoin wiki, etc...)
So, the FSF would also like to revoke their old public address, because they don't wan't to play the "who will spend new credits first" game with the thieves.

The problem is that there is no revocation procedure available in Bitcoin.
Do you think such a feature would be useful?

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"Half-baked" implementation proposal:
A special revocation transaction containing public keys (and signed with associated private keys) that can be included in a block. Miners understand those transactions and are forced to reject any following transaction that could be unlocked by those keypairs. (Note: this work only for standard tx)

[A]

Would the current script system allow conditional transactions requiring a signed certificate from a third party referee?
Probably it is not possible, for the same reasons that have been disscussed in this thread http://bitcointalk.org/index.php?topic=1786.0.

Though, it would allow very interesting uses of bitcoins...

Let's say A (Alice) is interested in some fancy item (lets say a fancynickname@freenode account) currently accorded to B (Bob) by C (Freenode).
So Alice would like to buy Bob account for 1BTC, but she is reluctant to send him the coin because she don't trust him. He might not give up his account even though he has received payment.
Of course Bob is also reluctant to give up his account before he received payment.

Now, let's say Freenode (who owns a GPG Key) agrees to sign account transfer certificates.
Alice would like to send Bob 1BTC conditional to the issue of such a certificate (within some deadline...).

And it would be so nice if such trades could be done!
A and B can trade without knowing each other, and both only need to trust the referee C.

Any thoughts?