Pieter, Matt and I also agreed that for maximum impact we should really try to ship payment protocol support in at least two clients simultaneously and ideally with a big merchant signed up too - to send a powerful message that we really mean it. Someone volunteered last week to do it for bitcoinj and if he doesn't pull through, I have some old code from EOY 2012 that I could update to the latest spec and ship at least some basic support. I'd hope that we can get Bitcoin Wallet or MultiBit updates out once bcj has support pretty fast.