A [p]atch to the standard bitcoin client would be great.
I'm not generally in favor of this at this time. "Green addresses" add irrelevant bloat to the block chain (no one but the recipient cares about that data, certainly not people 10 years from now) and compromise the privacy of Bitcoin users— even people who aren't directly the users of green addresses. They also encourage poor key management practices (e.g. keep all your funds on a single green address) or even more chain bloat (by doing two step transactions).
I also understand from talking to Magicaltux that green addresses have not been much of a success.
We added the signmessage functionality so that people could use a well known public key (your green address if you will) to sign any data they want, including transaction IDs, without putting anything in the blockchain or publishing it. For example, when you made a withdraw from a trusted bank, the bank could use its well known key to signmessage the relevant transaction ID. It could then either send this signature directly to the recipient if you provided a payment URL, or provide it to the user as a "confirmation value" which they could carry over to the other side.
Unfortunately MTGOX did not implement the former because they don't want their customers telling them that they're sending funds to $QUESTIONABLE_PLACE, and didn't implement the latter because it's not fully automatic.