This one is right up my alley! Years ago when I began using the trailcameras they were 35mm models and I rarely caught the same big buck twice in the same spot. Most times, just one shot and that was it. Of course, at that time, it was all we had we did not want to "believe" that the camera scared them. But, I think it mearly made them avoid the area, just like Ghost suggested.
Now that we have digital trail cameras, I am getting an entirely clearer picture of what happens. Again, as Ghost said, the 35mm film advance was definitely the culprit, and I say that with confidence. They just plain old did not like it. And I am talking about mature bucks, not necessarily does and immature bucks. My digitals are silent and I have countless pix of the same mature bucks both night and day. The flash, in my opinion, does little to effect them. I had a mature buck stand in a scrape for 5 minutes while my camera snapped picture after picture with the flash....no bother.
I too am curious about the IR cameras out today. I will definitely have one or two this summer to add to my arsenal of cameras. This whole camera thing has been a blessing and confidence builder in todays deer hunting world. Anyone who does not use them is absolutely missing out on some of the funnest most productive thing going on today!