Same thing is happening to my phone right now. (Power button won't function and auto starts when battery is put in. Before this, it would randomly reboot, but since the button stopped functioning it just stays on as long as it has juice in the battery/plugged in.) Thinking about saying screw it and going to the HTC Evo Design 4G, been a good last year with the prevail.
My guess is the power button is defective somehow, because I haven't installed, adjusted, or otherwise changed anything in a few days (and c'mon software wouldn't start the phone after a battery pull) and it just randomly started doing it, hadn't even been dropped or anything. Nor does the button seem to be stuck down in anyway, have tried many things to get it "unstuck".
To add to the problem, in my case, the USB port on the phone hasn't been able to transfer data for a few weeks now so couldn't just Odin to stock and get it fixed. (Oddly in CTMod USB settings are nowhere to be found, nor does it detect it's been plugged into a computer. But in HalfAssRom 2.0 it has the settings and detects, but the computer it's plugged into reports the device has malfunctioned, and no it's not drivers or the data cable for anyone who might comment about that. Friends prevail's and data cables all work on all my computers but not with my phone so it's certainly the phone itself, either KoumaKernal or the port itself(most likely) that's not working.)
Well, this is much longer than I had planned. lol
