The EFS 17-85 is designed to take into account the digital image chip of the camera and only specifically fits those cameras that are designed for the EFS lenses. If you tried to use it on cameras that it wasn't designed for you might damage the lens.

The 28-135 IS lens is a good lens especially as a walk around lens and is very good in my experience with this lens but that is on a 10D, should be similar on 20D as well. A friend of mine uses it as his lens for weddings but his is a film camera but along the same lines as the 10D (an EOS30/Elan 7 camera) and gets very nice images with it and makes a good living from his photography. The only problem wiht the 28-135 lens is that it stops down only to f3.5 so you might need something that goes to f/1.8 like a 50mm f/1.8 lens which would be equivalent to 80mm on a 20D, good for portraiture head/shoulder shots and would give a good blurred background as well. Something that is a little harder on teh 28-135 lens at its widest aperture, in my experience.

Kathy is correct in saying that the norm for portraiture lenses seems to fall in the range of 80-135mm lenses but a lot use prime lenses, i.e. an 80mm lens, 100mm lens or 135mm lens, but others use the zoom lenses as they seem to give a better aspect to the photo, allowing to work from a further away distance but being able to zoom in closer to the subject without the distortion of some lenses.