Is it damaging to the shoulders to do lat pulls behind the head?

No.  Why do you ask?

For years, I've done them both ways but mainly in front.  Doing them behind would always result in some slight shoulder discomfort, particularly when I began to fatigue and had to struggle to complete the last few reps.  You know how you tend to lean a little forward when you start to fatigue and you're doing them behind your head?  I guess I can really feel it pull in my shoulders and was worried about that.

You know what they say, if you want to be an old weightlifter you must first be a careful weightlifter.  

It can be damaging depending on your personal flexibility. It is a stress inducing movement on the rotator cuff. Normally it will not cause you any problems if you don't go overboard and overdo the weight/reps/frequency.

If you do feel something "wrong" while doing them behind the head, cease fire immeadiately. Otherwise, you know your own body and I would say lat pull away...

Edited to add: If you are feeling something on the last reps of a set, particularly when you are really working to complete the rep, I would say don't risk it. The shoulders are pretty fragile and easily injured. Lat pull downs to the front are much less stress inducing to the shoulders. I personally would try those if you are feeling something when doing them to the rear. A shoulder tear can really throw the kibosh on your whole upper body routine.

Behind the head pulldowns don't fully contract the Lats anyway, you are just placing more stress on the shoulders.

Not only can they damage your shoulder joints, pull ligaments, etc. when done behind the head, but they put tremendous strain on your neck. Think about it.... You are pulling a lot of weight behind your neck, with your head bent down in an unatural position, your body tilted forward slightly away from the weight. You then pull the weight down and compress all of the neck muscles in the process.

DON'T do it!!!

Besides, doing them behind the neck offers no chance of full contraction of the lats anyway, so whats the point?

Most people don't do a lat pull down correctly either. They just start yanking the weight using back, shoulder, arms, etc. Lighten the weight, START the move using only your back muscles, then pull through the move. Very slow movement, very strict movement. You will be shocked at how it really works when done correctly.