I'm convinced that Arduino will serve as a platform for hardware hacking. Most of my best learning has come from solving my own problems, though this time I don't want to limit it to just that.
Principles:
when feasible (which should be always), release all code as open source
do something useful
avoid, if possible, using the device to sense something then Tweet (that's pretty obvious)
avoid, if possible, having GPS provide the sensory data for the applications (that's pretty obvious)