There are also 2 types of fisheye lenses. Those that give you a circular photo, and then there are those that will give you a full frame photo.
Lenses such as the Peleng 8mm fisheye, are at the extreme end, and give you a nice round image, and thus fall into the first category.
However, on the 1.6 crop factor digital cameras, the 8mm circular image is cropped leaving a nearly rectangular image with clipped corners. This still is a much wider view than the same camera gets with the 16. As far as I know, the only way to get a circular image on a 10D/300D would be to use a cheap auxilliary fisheye adapter.