Will it eventually stop or will it keep rolling until you stop it?
At this point, it's probably either the parking pawl or the tooth is rests on is broke or worn. This generally happens from people who rely on the transmission's "Park" to hold the vehicle in place instead of supplementing it with the, wait for it, Parking Brake. There's a good chance there's nothing else wrong with the transmission. Be sure your parking brake is working properly and I would even consider carrying around a cheap wheel chock for those super hilly parking spots just to be safe.
At least, that's my opinion at this time.
Oh, or trade it for a Vibe w/a stick. ;-)