This does not prevent theft, but is more PIC related that the
prvention suggestions.

If you don't mind running some wire along the runways, why not
put a photocell next to each bulb so that you can detect
darkness, daylight and bulb on levels, use 1 PIC for each 8
lamps and network them to one master PIC that can monitor the
state of the power for lamps and turn them on and off. Attach
a cheap modem to the master PIC and you have a complete control
circuit which can be made to monitor all lamps, test them and
have slave pics calibrate themselves under software control.
You can have it to dial up some other system and send lamp
status for a whole field periodically, give alarm if more
than one lamp goes out in a given time period. Master could
have one photocell which it monitors to detect the brightest
daylight and darkest nighttime to calibrate the lamps automatically
every day. If you have some free code space, you can also add a
dial-up server that would allow some other system poll your lights
monitor for lamp status and draw a runway map with the real light
status.

If you make up your network so that each PIC is a repeater for next
one, you can run it to great lengths and also have ability to detect if
any of these is broken or stolen.

I don't know much about your airfield, maybe it's a huge overkill but
it would sure be fun to build :)