Remote Wipe Feature in Android 2.2
Your server has to be able to communicate with the phone somehow to send the wipe message, either by a phone network connection, or if it's connected to one of its saved Wifi connections. Not 100% sure about the Android implementation, but normally the message will queue up and be received when the phone comes into coverage. The phone also has to be switched on at some point, or it'll never be received.

This is a limitation of all phones that have remote wipe capability (eg Windows Phone, Blackberry, etc). It can't send the signal by magic!