This condition plugin for Locale and Tasker detects what type of activity the user is engaged in. The different kinds of movement that can be detected are walking/running, riding a bicycle, driving or riding in a vehicle, and staying still. It uses the new Google Play APIs just released at Google I/O, which are designed to be low power usage and will improve their detection over time.
This plugin is useful to do things like turn up your media volume while you jog or bike, mute your ringer while you're sitting still, or remind yourself not to text while you're driving. Note that the detection is not instantaneous to keep battery usage low; activity updates happen about every 30 seconds.
Important: This is only a plug-in. It requires either "Locale" or "Tasker" from the Google Play store.