Thinking more about bumptunes, as I sit here with my PowerBook balanced on my lap: My hands are always on the keyboard, so a good interface to jump around tracks would be to rap sharply on the right with the base of my palm to go to the next track, and knock the left to go to the previous one. Because my legs are crossed, pushing my knee up is actually quite a careful and controlled movement, and so that forwards/back tilt is more suitable to a one-way adjustment... say, scrolling down in whatever application is active.
Also, the interface shouldn't look at absolute tilt--it should look at changes, and the rate of change. A sharp tilt to the right which goes back to normal quickly should advance to the next song, but a slow tilt which continues for a while should fast-forward (perhaps). I also quite like the idea of pattern matching, like with gestures: a double knock is like a "can I come in?" so maybe that should sign you in to IM. And, like gestures, tilt/knocking is not a digital action (like a button: press it or not), it's analogue. A soft knock could start iTunes player with the volume low; a hard knock could start it with the volume high. And then there's tapping rhythms... so much potential! And I don't have a PowerBook capable of it to play around with, gah!
What I love about embodied interaction is that the rules of thumb haven't been discovered, so you find out what's best by making it work and trying things out. It feels so natural.