I fixed this yesterday, using the solution posted here:
http://forum.appliancepartspros.com/microwave-repair/4882-advantium-turntable-light-always-main-relay.html
Unplug the unit.
Replace the interior light using these instructions:
http://www.instructables.com/id/GE-Advantium-Microwave-Halogen-Lightbulb-Replaceme/ The inside of my unit looks a bit different from the pictures, but the procedure is basically the same.
Additional info not in the procedure link above:
Remove the one screw at the top of the front control panel.
Slide the control panel up to disengage it, and pull out slowly (has wires on back)
There are four black rectangular things running top-to-bottom on the PCB board on the back. One of them is labeled RY1.
Tap on RY1 with a screwdriver a few times (very high tech). I actually tapped on all of them for good measure.
Reassemble, see if everything works now.
It took me about 30 minutes for the bulb and the control panel 'adjustment'. Everything is working now.