Kevin,
Alright, I'm not an expert or anything, but I'll take a guess.
First, the reason you can't unlock the driver's door and the reason you can unlock it with the passenger door are two different things. The driver side door is being locked/unlocked via the key. Unlocking the driver side door with the passenger side is done via the locking/unlocking solenoid. So one does not have much to do with another.
Second, what you seem to be describing is a tumbler beaking or falling out of the lock. Once again, my experience is not with the kind of keys that we have for our Porsches, but I have to guess that the concepts are not that different.
On your house key, it has grooves at different heights. You stick it into the lock, and it moves little spring loaded tumbers to the correct height and allows the lock to turn. It sounds like one of the spring loaded tumbers fell out keeping you from making the correct contact with the key. Now, I assume that this isn't affecting your ability to start the car, just to lock it from the driver's side door.
Either way, this doesn't sound good and it sounds like it might cost a bit. Assuming that they can't just replace one driver's side lock, you may have to replace 2 door locks, and the ignition switch. I bet that isn't going to be too cheap.
This is just a guess. But from what you've described, its an "educated" guess
