I may be able to shed a little light on this subject, but need a little help too.
I have been having the same problem for a while now, and it's starting to get irritating to say the least. The problem seems to revolve around a sensor in the shifter mechanism that tells the vehicle that the engine is running or not. For whatever reason, the sensor doesn't report the correct information, and the key won't return to "off" and release the key. You will also find that when the sensor is not working correctly, the sunroof will not operate. I have had the car (08 GT) at my dealership three times now, with the ignition barrel replaced the first time (did nothing), the sensor replaced the second time (took two weeks for it to start again), and this time (today) will be to look for other alternatives.
When the sensor decides to work, you can hear an audible click from the ignition when the pin releases that stops the key from moving to "off". With the sensor in the shifter, moving the car in and out of gear (even rowing through the gears in manual mode) seems to have the most success in getting it to release. Once the sensor was replaced, it will release the key a lot quicker than before. It used to take upwards of 5 minutes of monkeying with the car to get it to release, but now it will cough it up in about 30 seconds. Still shouldn't be doing that though.
So, knowing this now, does anyone have any additional insight?
In regards to the additional info that has been posted previously in this thread, I don't have anything tapped in on fuses, and all are in tact.