This other thread might not be as visible, being where it is:
http://www.droidforums.net/forum/motorola-milestone/22898-slider-slightly-loose.html
The question I asked there is: Does your phone make clicking noises when touching the screen on the half of the screen indicated in the screenshot? (portrait mode, left half of screen)
For those whose previous phone was loose....was this the reason for the return?
If enough have no noise I guess I'll bring it in and swap it out again. It is good to hear that if the slider is firm it stays that way. There doesn't seem to be a concern it will act up 1.5 years from now.