A week after I purchased my new HTC Incredible, the power button would fail to turn my phone off most of the time.
Upon removing the rear battery cover, the button worked just fine. I realized the problem was not the power button, but the opening for it. As the front and rear covers were designed to come together (i.e., form the front and rear button opening), they squeezed together too tightly on the button. This interfered with button movement.
I took the rear phone cover and gently filed away much less than 1mm (around the opening) using a diamond-tipped microdermabrasion device. The amount of plastic removed was so minimal, no can tell by visual inspection. This fix worked like a charm; I have been using my phone for days now without any problem recurrence.