.... In fact, using the center point aimed directly
at a face or chest seems to be softer than some areas outside of the
center point.
Are you saying that when you use the center AF point, aimed directly at the subject's face, the focus is off? Or are you saying that no matter what, the center of the image is soft compared to outside areas?
If there is a part of the image which is always soft no matter how you focus, then I'd wonder if there might be something on the front sensor filter that's acting as a "smudge" of some kind and blurring everything under it no matter how you focus.
On the other hand, if you're just saying that AF is off, then that's a more common issue, I'd think.
If it's the latter, have you tried manually focusing? If so, what were the results?
And have you tried testing the AF by shooting with the camera on a tripod, at a fast shutter speed, at an object with good contrast like a calendar hanging on a wall or the like? If so, are those shots coming out soft too?
If it seems that the AF is out of calibration, I'd be tempted to try another body if your shop is willing to exchange the camera. Or you could send it in to Canon to be calibrated. The AF should be very good. I'm finding excellent AF with my 40D, so it's disappointing to hear of other people having problems.
If it's more like a softness issue that's not focus-related, then that does make me wonder if there's some contamination on the sensor filter. And if that's the case, then again, I'd be tempted to try another 40D from the shop if they'll do that for you. And if not, then the camera should go back to Canon for their assessment and repair (cleaning?).
I've noticed that there are some smudges on my 40D's mirror, but they don't affect the images, of course. I wonder if they've had some problems with lubricant getting where it's not supposed to be!
I can say that if my 40D is any indication, then you should be expecting and getting very good AF performance. For me, the 40D makes my 85mm f/1.8 useful wide open where it was always something of a disappointment on my 20D.
--
Jim H.