I recently returned a new 24-105L to Canon because of soft images. Quality was distinctly inferior to 3 other lenses I own (Canon 28-135 IS, Canon 70-300 IS, and Sigma 17-70). All of these are decent lenses, but I don't think anyone would expect them to compare favorably with the 24-105L (with the possible exception of the 70-300 sometimes). Canon returned it, said they checked and cleaned it (it was brand new, wasn't "dirty") and if the problem recurred to return it with the camera body. But the camera body works fine with the other lenses. Is there any reason to think there is some issue with the body (Rebel XSi/450D) that would cause problems with this lens and not another? Or is this just Canon not wanting to replace an obvious lemon of a lens? Any insights appreciated.

Maybe needs to be calibrated to YOUR body?
have you tried that 24-105 on another body?
24-105 I own is one of my sharpest lenses, if not THE sharpest.

Sounds correct, I gander that Canon can do the calibration to accomodate the distance tolerances between bodies and set the lens to your camera body. I once had a similar situation with a zoom lens that I sent in for calibration, and saw no progress until I sent it back a 2nd time with the body. Then focus was dead on, but I don't understand why thinking that focusing is a closed feedback system. Try this: shoot full aperture spot-focused at a mark on a surface at an acute angle to the camera, such as a yardstick at an angle, or even a brick wall. Then see if your focus mark is in the center of the DOF or if shifted back or forward. That way you will be able to separate out "sharp" from "focused". My 24-105 is quite sharp.

Is this what "micro-adjustment" resolves in the newer bodies, i.e. 5DII?

I have done a test for focus: I lined up 5 bricks about 2 feet apart, then with camera on tripod (center AF point selected, IS off) about 15 feet from the closest brick, shot down the line, focused on the center brick. Did this at various apertures and focal lengths. In all cases, the center brick was in fact the sharpest point (just still not very sharp). If this was a front- or back-focus problem, wouldn't the sharpest spot be somewhere ahead or behind the point I focused on? Or am I misunderstanding something?