Ok, not sure anybody is still here (or that anybody is left who hasn't driven over their phone with a truck!) I have this very specific chain of events which happens to my phone.
When things are going normally, my battery will only go down by about 1/2 in about 16 hours (I leave it plugged in overnight.) But then, at random intervals, the phone enters a demonically-possessed state.
-- Without me touching it, the screen turns on, and stays on.
-- The phone starts heating up, and within a few minutes is so hot that if I were to plug it in the power management system would give me a message saying that it can't start charging the battery until it cools down some. It's unpleasant to hold or to have in my pocket.
-- When I click on the power button, the phone takes a screen shot. Makes the camera noise, and the screen shot appears in the gallery.
-- The battery discharges rapidly (well that's not unexpected, that's physics, since the heat and light come from the battery.) It will go from 50% to zero in about a 1/2 hour.
-- If someone calls me, the phone acts like it is off. The call goes immediately to voicemail without ringing, and there is no sign of the missed call on the phone.
-- Hitting the Back button repeatedly will get the phone out of the screen-shot mode and allow me to turn it off, but often I have to completely power off the phone to get it to cool down.
If I go to Settings->Applications->Battery Usage, I can see it all its graphical glory. The little graph at the top shows the phone just happily going along, with the charge very slowly going down, and then there is a sharp corner and a dive. The "Display" item goes up from its normal 2-5% to 30-40-50-80%. If I catch it very early, I can power off and back on, and then another sharp corner on the graph and it goes back to the charge dropping slowly and staying very flat, and the Display battery usage steadily falls while the reasonable things (cell standby, wireless) climb back up in relative position.
Oh, and has been pointed out, the real way to get a screen shot is to press-and-hold the Back button and then hit the power button as a "shutter." I've used it to take lots of examples of the Battery usage when the phone is fine, and the battery usage when it's in screenshot camera state.
The rest of you folks are not mentioning the part about battery charge converting to heat when you talk about your phones entering screenshot camera state. Is this because it's only happening to me? Or you just haven't been putting all of the pieces together? I've had the phone for almost a year (read warranty going to run out) and am trying to figure out how I should beat up on the US Cellular people to get it fixed or replaced...
The thing with the Back button that's suspicious is that if a hardware glitch (like an electrical short in the capacitive screen) were causing the Back button to engage like it is being held down, then that would turn the power button into the screenshot shutter. You know, as in "it's broken!" And a fixed or replaced phone might be just fine.