Ram and I were talking about my former car earlier this afternoon, and I remembered something. A lot of people in the club have bought S2000s since I had this problem, so I figured I should pass it along.
Using the remote key fob, you have to press the unlock button twice to get the passenger door to unlock. There's a timing to it - doing it rapidly like double clicking a mouse often won't work. Because of this, I got into the habit of thumbing the unlock button repeatedly, as I walked up to the car, to "make sure" it was unlocked.
Don't do that.
The reason is, when you push that button, the driver side lock motor fires each time, whereas the passenger side motor only fires every other time. The lock motor is attached to a bunch of rods and bellcranks that move things like the lock/unlock lever inside the car - you know, the thing you flip if you're driving through a bad part of town and want to lock the doors.

Well, if the door is already unlocked, when you thumb the unlock button on the remote, the motor is tugging on the lock/unlock lever. But this lever is already flipped to unlocked, so it doesn't move. This fatigues the plastic on that lever, and eventually, can break it.