In-app billing for Android is a pretty simple thing, really. Say you're playing a game and want to buy a new level, song, weapon -- whatever. All it takes is a couple apps, and you've got the new level, song, weapon -- whatever.
We gave it a go on Tap Tap Revenge -- you should remember our early look at it back at the Honeycomb event at Google HQ. And indeed, it's simple enough. Tap on the song we wanted to purchase, confirm the purchase, and we're on our way. (That was the easy part -- Tap Tap Revenge itself is something of a confounding application, or maybe we're just old and couldn't handle all the flashing lights.)
Anyhoo, look for in-app billing to really take off in the coming months.