The strut mount is actually two components -- the metallic part that's obviously bolted to the fender, but also a rubber spring seat and strut piston sleeve that is inserted inside of a bearing in the metallic part. The bearing allows the strut assembly to rotate, and like all bearings it can wear out. I just pulled mine after an estimated 120k mi and the bearing still moves, but it's a little noisy. Worse, the rubber seat has a metal sleeve in the center that the strut piston is bolted through; on my car the sleeve actually separated from the rubber seat, allowing the shaft of the strut to move at will. (Finally, the strut itself was so worn that I was able to compress the piston all the way into it, and then it simply stayed there without rebounding out as I chucked it into the dumpster.)

If you take a close look underneath the large rubber-reinforced washer underneath the main nut on the strut shaft, you may be able to see if the shaft sleeve has separated from the rubber mount. This is an easy way to check it without disassembly.

As I said before, I'll upload some photos soon.