I don't know if this is a general problem, or if I just got "lucky". I basically turned my 40D into a paper weight by using the "Clear all camera settings" option. Here's what happened.
After I attached my 580EX flash and then removed it, the automatic built in flash wouldn't work even in "auto" mode. I assumed it was some sort of internal setting that I couldn't find in the menus, so I selected "Clear all camera settings" on the yellow "three dot" menu. The good news, this seemed to clear the flash problem, the bad news, no pictures. I don't mean that it erased my memory card, the camera would no longer capture any useful images. They are all black, as though the sensor has been turned off or worse.
When shooting a photo, everything seems to work as expected. Autofocus, aperature, or shutter speed selection, mirror "click", review after the shot, etc. but the resulting image is as though I have my lens cap on. I even tried "live view mode" and the LCD is black except for the white rectangle. I hear the mirror lock up out of the way, but there is no image. I tried everything to get it back to a functioning condition, but nothing worked. I had v1.0.4 firmware and upgraded it to v1.0.5, but that didn't help either (I didn't expect it to based on the description of v1.0.5, but I was hopeful).
I called Canon support and they walked me through a few things. Pretty much the same things I had already tried, and now I get to send it in for "repairs".
If anyone has experienced this, or knows of some obscure setting that gets set/cleared as a result of Clearing all camera settings, I would really appreciate some help. I don't want to be without this camera for 3+ weeks.