For the sake of any future Googlers: If you use Firewire Target Disk Mode to transfer everything from your iBook to your iMac, you may inadvertently transfer over the setting that displays your battery status. As an iMac has no battery, you will thus be unable to make the stupid indicator disappear. After much googling to no avail, Snookums helped me search through my preferences and find the relevant line in the “com.apple.systemuiserver.plist” file. Just delete that, logout, and when you login again the indicator will be gone!
