I am not sure whether it is the camera or the lens at this point. I do think that the 7D has gotten a bad rap primarily due to it bump in resolution. Lenses that looked tack sharp on a 10mp camera may not look so sharp on an 18mp camera.
I think I am going to cry. This is a huge misconception that gets passed from thread to thread and refuses to die.
A lens that looks tack sharp on a 10mp camera will show exactly the same sharpness on an 18mp camera (barring calibration differences between the cameras). If you look at photos side by side, viewed at the same size, if anything the 7D will be slightly sharper.
What confuses beginners is that the photos will look different when viewed at 100% on a computer monitor. The 18mp image has 80% more pixels, so at 100%, the image is physically 34% wider and 34% taller. When you physically enlarge any photographic image, you enlarge the blur. So the bigger image looks less sharp to these people who don't quite realize what they are doing. This is about geometry, not technology.
Of course this has no effect on the photos in real life. In real life we do not enlarge our photos more, merely because we bought a camera with more megapixels. I did not buy larger paper for my printer when I upgraded to a 7D. I did not buy a larger monitor. So the photos are exactly the same sharpness as my lower resolution camera.
For instance, my 100-400L seemed great on the 20D (across its full zoom range), but when using it on the 7D it became clear that it back focuses at 100mm and slightly front focuses at 400mm. I MAed so that it focuses better at the longer range (since that is what I use more). But if I send in my other lens (and possibly the 7D), I'll probably send in the 100-400L too.
It is easier to detect minute focus errors when you have higher resolution because you have more pixels available, and hence it is easier to enlarge the image. As mentioned before, blur is enlarged when the image size is enlarged. But if the focus inaccuracy was not noticeable at a given image size on your 20D (e.g. an 8.5"x11" print), it's not noticeable on the 7D at the same size either. This is only a problem for you at 100% on your monitor, not in real life. That being said, it is also possible that calibration differences are resulting in a situation where the focus is legitimately off on the 7D compared to the 20D, but that is not due to the resolution difference. MA or a factory adjustment can fix this.
