This is a quick hack to let Locale detect whether an Android device is plugged into USB or some other power source. Requires Android 2.2+. I wrote it because I wanted to detect my car holder, which is the only place I charge my device that is not USB. Perhaps someone else will find it useful. If you want to be more specific than the "AC/dock" option, there are at least two plugins from other people that detect docks.
For reference, here is the BatteryManager API I'm using. Notice how there are only values defined for battery, AC (which includes other non-USB power sources), USB, and wireless. I've not seen other values from any kind of dock; there's a separate API for docks.
If you're using Tasker instead of Locale, you don't need this; it's built-in.