It just happened again. Since last night's incident, I turned off the car today at least 9 times without incident.

I did have my phone with me just now so I did take a video; here's the link (if you have the sound up you can hear the engine changing pitch):

http://www.vimeo.com/8447749

I did figure out that just can't be out of gear but it has to be in "reverse" in order for the car to turn off, tried it in "drive" and it kept going.