You can also get switches that mount in the door jam that turn on the lights when the door is opened.

If the switch is inside the closet, then you can replace it with a motion detector. Then it won't care if the door is open or not just whether something is moving near the sensor.

If the switch is elsewhere, you could put a zwave motion detector inside and let it control a zwave switch.

Z-Wave

Yes, sorry. Too early in the morning when I answered that. Z-Wave is a multivendor home automation standard. Neat thing about it is that it is very low power (as far as the communications is concerned). You can put battery powered motion detectors in anywhere (you can stick them up with double sided tape) and have them control a Z-Wave switch in the remote switch box.

I have a motion sensor in my pantry. It is one that has a remote sensor (about 18 inches away) and a screw-in base to a standard light socket. When I open the door, it turns on. Turns off a few minutes after I close the door or walk away. It just happens that the socket is a pull-chain, but a wall switch would have been fine.

Here's a sample:

motion sensor for light socket