I got my 5DM2 in February. I shot three weddings with it and all was well. On the fourth wedding, a bright red dot (blown pixel/cluster) began appearing on all of my shots, in the same place.
Unfortunately, I could not see the red spot on the camera's LCD, and even worse, I did not discover the problem until the following week - AFTER I'd shot ANOTHER wedding. 
It took me about twice as long to do my regular RAW > JPG conversion with the shots from the first of the affected weddings, as I had to fix the bad pixel(s) manually. Lightroom's dust removal tool worked great for this task, by the way, but it still took more time than I wanted to spend. I have yet to tackle the shots from the second event, but will be doing so this week.
Anyway, on Wednesday afternoon last week, I dropped off the camera at the Canon service center in Jamesburg, NJ, getting there just before they closed for the day. They assured me that as long as there was no water or impact damage, that it would be repaired under warranty.
On Friday morning, around 11am, my FedEx guy showed up...with a box from Canon. Now THAT was fast service; I was very impressed!
Incidentally, that was the first sensor-related problem I've ever had with a Canon body. I'd guess I've shot well over a million frames on Canon DSLR's since 2002, on a D60, 10D, 1D, 1DM2, a pair of 1DM3's, and my current 5DM2 and 7D tandem.
