The Canon 1D Mark III camera has a problem (its actually a set of serial number ranges that do). Long story short, in some circumstances AI Servo auto-focus performance is poor, if you want the details check out Rob Galbraith's article (he's the one who first described the issues with pre-production bodies). When the fix was announced back in November I called Canon and got my body on the list. Upon returning from the holiday break, I got a message from Canon that it was my body's turn. They very quickly sent out a UPS label for overnight shipping and yesterday the body was packed up and dropped off at the UPS store. Folks on various forums have said they got their bodies back in less than a week, so I'm hoping for the same luck.
Have I noticed any issues with AI Servo? Its a difficult question to answer since I never used AI Servo in the past (you don't need it for shooting landscapes, buildings or anything that doesn't move) and I only recently started using it for birds. When I first started shooting birds with the 500mm lens and the 1D3 I didn't get a lot of keepers, but I'm sure that is because of my error and relative inexperience rather than a faulty camera. Since I did do a little bit of bird photography with the 1Ds2, I compared my 1D3 images versus the 1Ds2 ones and found that the keeper rate was fairly consistent. Last week when I was photographing my sister and Kim at Bower Ponds I used AI Servo exclusively and found many shots were out of focus. I think the shutter speeds were high enough but again its more likely it was user error.
Until the 1D3 comes back, I'll be shooting with Kim's Rebel XTi. It feels really weird working with the camera after using mainly the 1D3 for several months. I also forgot that the Rebel series of cameras don't have an external sync. port so in order to fire my strobes I had to use the onboard flash to trigger the strobes which results in harsher lighting than I was aiming for.
Technical Specs: Canon Rebel XTi, 60mm macro, f/8, 1/10s, ISO 200, strobe fired.