neuroanatomist: Thanks for all the info and explanations. I just did some more test shots - and you may be right! It seems that there is a slight front focus issue - noticeable only at very wide f stops and "masked" on consumer lenses.
I would like to upgrade the body - however at this point in time I would wait to see what new bodies appear then purchase the 7d - a 2 year old camera - which is fundamentally my 550d in a fancier package. I'm not a pro and I need to watch the budget.
Makes sense. Actually, AFMA was one driver for me when I went from a T1i/500D to the 7D (mainly I was looking for better AF, but I had recently gotten a 100mm f/2.8L Macro IS and it was backfocusing a bit on my T1i - I lived with it for a while, but for macro it's not a big deal as that's usually shot with manual focus/10x Live View).
What's the normal tolerance on phase-detection AF? I just bought a refurb 24-105 that I've been trying to test. It might be slightly off focus -- maybe 2-3 mm at f4. I'm still trying to improve my test technique, though, so it's possible it's experimental error.
The tolerance is not stated in terms of depth of field. It's stated in terms of depth of focus, which is analogous to depth of field, but on the image side (i.e. focus in front of or behind the image sensor in the camera, usually measured in Âµm). The tolerance is 'within one depth of focus for the lens at max aperture' for standard precision sensors, and within 1/3 of the depth of focus at max aperture' for the high-precision center AF point when activated by an lens of f/2.8 or faster (or f/4 on 1-series bodies). If your body has AFMA, each 'unit' of AMFA equates to 1/8 of the depth of focus at max aperture.
For normal subjects (i.e. not macro), depth of focus is dependent primarily on aperture, and relatively independent of subject distance. However, depth of field is affected by subject distance, so with close subjects, slight focus errors become more apparent (because depth of field is getting relatively shallower, whereas the specified tolerance for depth of focus is not).
Honestly, the best way to test AF is with one of the commercial tools that I mentioned above. Personally, I use a LensAlign Pro.