I'm going to leave this double post for posterity.
Next time I get asked why transactional web-apps (apps that send stuff and generally do things rather than just pull stuff together) cannot be built using all that cool, simple, mashup stuff?
The two posts referenced above is basically the why. For mashing up, you ask me stuff, you don't get it, you just ask again. Simple. If I then respond to your first request some time later, no harm done. Just throw it away.
but if I *send* you stuff and I'm not sure if you got it...