It took me a little while to catch on - but my 30D back focuses consistently, sometimes more than others. In most situations, its fairly mild - and examining the resulting files just led me to believe that the 30D naturally produced slightly softer images than my previous 10D did... Then I stuck a polarizer on it for the first time on my 17-40... and the camera massively back focused. Tried the same filter on my 70-200/2.8, same problem but not as severe. I then noticed that, without the filter, both lenses were still backfocusing, but not quite as much as w/ the polarizer on them. A slight tweak on the focus ring to set the focus closer made them noticeably sharper in the viewfinder (the difference between getting that "cruncy/gritty" in focus look in the viewfinder and not getting it). It also does the same thing w/ my 50/1.4.
The 10D didn't have this problem at all. It should be noted that I rented a 70-200/2.8 for an event so that my wife could drive a camera for me, as well - the 30D wouldn't focus at all w/ the rental lens, but the 10D had no issues at all.
I have checked the position of the focus sensors - this doesn't appear to be that issue. And... yes, I know how to use the focus on the camera
So, is there a consensus on which Canon service center is the right one to send the camera to? The camera is still under warranty, luckily - hopefully this can get corrected w/o incurring a cost... I remember reading stories of differing levels of skill in fixing focus issues at different service centers, etc. I can't seem to find good info on this, right now - can't find where'd I'd read what in the past. The 10D I've got is so solid, I just stopped thinking about those sorts of issues...
Any hints on navigating the warranty service maze, etc, or info on what to expect? This is the first time I've had to make a warranty claim on a piece of photo gear...