Just had this same problem. Since the phone was still under warranty, Verizon replaced it. Not sure if it's a repairable thing.
HTC devices has this issue because how often the button is used, if you have insurance it is easier to have it replaced because the only other solution is to brake device down and repair the button plunger which will void the warranty.
I'm having the same problem with my phone. The button was raised when I got the phone six months ago, but now it is flush with the top of the phone making it difficult to turn on. I talked to Verizon Customer Service about it and the dimwitted rep told me that this was "a design feature." A design feature that makes it difficult to turn the phone on and off? When I reported that the button was raised higher when I got the phone, but that it had regressed I received the same response. Then I informed her that there were reports on Verizon's support boards (and other HTC forums) about this problem and she told me "you can't believe what people post online."
I"ve never known Verizon to be known for customer service or support, but this takes the cake. My phone is still under warranty and Verizon won't do anything about it. Guess its time to file some consumer complaints.
I've always had this issue with my phone, too. It's so hard to turn my screen on and off because the power button is so flush with the phone body. I just want a different phone, but my upgrade date is so far away.