I already understood that with the current bitcoind client you can't send messages with your transactions because otherwise people would be using it but what it sounds like you're saying is that someone could create a patch for the bitcoin sourcecode which would add this functionality to bitcoin even though there are no plans from the developers to include a messaging feature. In the case where Gaven modified the source to include a vanity feature, someone else in the next half hour could modify the source to allow a PGP public signature to pass with the transaction and then anyone with that modified source would be able to see it or the intended recipient would also be able to see the PGP public key. And finally; this could be done without disrupting the Bitcoin network because you're using something that's already in the protocol?
It is possible. Each output can contain about 9kB of arbitrary data. Bitcoin won't relay/include non-standard transactions any more, though, so you'd have to create a block to get the transaction included in the block chain. You could also include a substantial fee with the transactions, which would encourage miners to change their rules.