Hmm. Have you tried resetting ALL the settings back to the factory settings? (it won;t affect your clock etc) If nothing yet, you might want to send it in to see what the deal is. Have you tried calling a Canon Rep? They might have an answer none of us know about. You are everyones eyes on what is going on, so be specific when you call them.
I have had that issue when my settings are all wrong, it won't shoot. So I (cheat) switch to A and see what the camera thinks it should be, go back to M or AV, and set closer to those settings. I ONLY do that when the camera won't shoot. You proly already tried all these things, but just in case.....worth a try.
Best of luck. I hope you can get it figured out. I am still curious about the solution when when you find it.
