I found a solution to a Volvo problem which saved me a trip to the dealer; maybe it will be useful to someone else, too.
My wife has a 2001 S-40 with an automatic transmission. Recently, she's had trouble getting the key out of the ignition. She'd stop, put it in park and turn off the engine but the key wouldn't budge. She'd first try turning the wheel, then would restart the car and back up then forward a bit until it finally let go of the key.
I shot some Lock-Ease aerosol graphite solution into the keyhole, which didn't help. Finally, I noticed that when the key was stuck, the park release "trigger" on the shift lever wasn't released, it was stuck in the "in" position. I shot some graphite around the edges of the "trigger" and kept working it until it moved freely. Problem solved.
My guess is that the car doesn't sense that it's in park until the "trigger" releases, flipping a switch somewhere in the shift mechanism to allow the key to be extracted.