Try Google and enter "automatic closet light". There's several ideas there.

Thanks jimisham, I did the search you mentioned but I keep getting *mechanical* spring switched lights, not motion sensor lights. I have young children and several large walk in closets. I don't want the lights staying on when the kids leave the closet doors open or worse, the lights going out if the door gets closed with a child inside. If you ask my kids about their favorite part of our new house they'll tell you, "the Harry Potter room under the stairs!"

Thanks for taking the time and making the suggestion. Even though the the automatic closet light switches won't work for us they're a great low cost solution and may work very well for others.

I thought I saw some in there that were motion detectors. I have seen motion detectors that screw into the light socket and the light bulb screw into that. I'm not sure how well one would work in a closet.
Try this search: light socket "motion detector"

I've had several of the detectors that replace the regular wall switch. I use them in a bathroom that's about the size of most walk in closets.

The first lasted several years, but each successive one lasted less and less time: I'm lucky to get a few months at this point.

I've no idea why, but its very frustrating.

I do have one that screws into the light socket, and its lasted the longest of any of them. It's not pretty, and I'm not sure what [if any] sort of decorative fixture you could use that would both fit, and not block the sensor.

I have this in my garage, and its been great out there...

We have florescent lights in all the closets so the "screw in" kind work for us either:-(

oruboris - what brand are you using? I want to stay away from what ever it is! Do you think the more expensive brands will last longer? With all the bathrooms, closets, laundry room and pantry I'm looking putting in 14 of these motion sensor switches and I want ones that will last!

How about this:

Here is a link that might be useful: Screw in motion sensor

I had the same idea since there is a code about not putting the switch inside the closet where we are building in N.C. Today I saw a 15" round fluorescent ceiling fixture with a built in motion detector. It was at Lowes, made by Portfolio for $48. Pretty neat idea I thought!