Thankfully, I read a post on Monday (3/10) about a somewhat common issue with the Canon 5D on my favorite digital photography forum (Digital Image Cafe). So, when I was photographing an event on Friday, heard a clunk and suddenly couldn't see through my viewfinder I knew exactly what had happened. The mirror had detached: http://www.usa.canon.com/consumer/controller?act=PgComSmModDisplayAct&fcategoryid=215&modelid=11933&keycode=2112&id=55659. This was my last image and the mirror had apparently already detached at this point:
I honestly don't know what I would have done if I hadn't read the forum post on Monday! I called Canon, they're sending a UPS label and repairing it at no charge.
Thanks to a fellow photographer and wonderful friend/person, I have a 5D to use while I'm waiting on mine to get repaired. So, if you have an appointment scheduled during March/April you have no need to worry!