Nothing could be simpler.
Completely invisible until a movable app is installed. Please don't comment that nothing happens.
Move to SD card
Tags: sdwatch, sdwatch.apk, sdwacth apk, running sdwatch, sdwatch apk, free sdwatch, sdwatch apps, using sdwatch.
Updated to support Gingerbread. There's no need to update if you aren't running Android 2.3 or later. (Other than to get rid of the update notification.)