The idea is to have the wildcard signature apply to all unspent transaction outputs (UTXOs) of a standard form and paid to a particular address, and be a signature of some kind of message to that effect. However, not all transactions will be able to match the wildcard, and only UTXOs in the formula of "OP_DUP OP_HASH160 [a hash corresponding to your address] OP_EQUALVERIFY OP_CHECKSIG" could potentially apply to it. While this idea breaks the programmatic nature of the requirement for spending bitcoins, Dave Scotese suggests that this could be justified by having a special extra mining fee for any transaction that used this opcode. There was also some discussion on this proposal in the bitcoin-wizards mailing list.