When Google released the updated Froyo software, they added a feature called remote wipe, and now this software has been enabled for you. Created by XDA member vesperaNovus, this software allows you to simply set some occasions where if applicable, your device will automatically wipe itself clean.

Android 2.2 (Froyo)+ includes a new administrator feature. The administrator must be enabled by the user before it can perform its actions. This application focuses on the memory wipe feature of the administrator. If your device gets lost or stolen you can set up this application to automatically wipe the memory in the following conditions: failed passwords to unlock screen, SMS password received, subscriber ID changed (SIM card).

Try it out, but remember, you need Froyo for it to work.