Ok, so I got a response from HTC who recommended I contact T Mobile for a warranty exchange. Before I dialed 611 to get that going I started the camera app one more time. No flashing.
Hmm, maybe it's related to battery temps somehow? I exercised the phone some, but still no problem with temps going from the initial 87 all the way to 102.
Ok maybe it was an app that's not currently loaded to memory? I just spent an hour and a half opening every last app on my phone and testing the camera after every one. It still works...
I'm hoping that whatever the issue was it's worked itself out, but I'm always suspicious of that kind of thing. My current wild theory is that the flash was caused when the camera focusing mechanism would stick, since the flash was accompanied by a click from the lens. Maybe a days' worth of being in my pocket and heat cycling worked that loose from its too tight "new" state...