I've taken apart a lot of electronic stuff as a hobbyist and electrical engineer. In my experience, a rattly button is usually the result of the plastic overlay, i.e. the button that you touch with your finger, not being precisely fit to the electrical switch underneath it. There is simply some physical space there in which the button moves and bangs against other plastic parts of the case making the rattling noise. It dos not imply reduced lifespan because the electrical switch under the button is a completely different, sealed mechanical entity on which the plastic button rests.
That doesn't mean that a rattly button would NEVER indicate something more serious going on, but it would be pretty unusual.
Yes, the buttons on my DX rattle a little if I shake it and it doesn't concern me. I believe it is just a part of the normal manufacturing tolerances.
I haven't seen a teardown of the DX and I have yet to put a screwdriver to mine, so I can't say for sure, but some of the rattling you hear may be due to the orientation sensor inside the device. Some of those sensors involve a little metal ball that fits inside a wire "cage". The ball is big enough to contact two wires at a time, hence an electrical switch, and that provides input to the display driver to tell it which way is up.