another reason to lock your carry-on
A few years ago I started locking my laptop case before placing it on the x-ray belt. I was mainly interested in keeping TSA employees from taking things out but I suppose it also makes it harder for them to put something in as well.
A side benefit is that since I started locking my case I haven't had a single "hand search" of my bag. Use to get one every 5 to 6 times through. Now I never get one.