if you're just looking to get some shots and aren't SUPER picky about image quality, the sigma fisheye would probably suit you just fine. the canon lens is a sick lens with great glass, but it's often $500-600 depending on where you look.
and the crop factor skis4x4 mentioned has to do with sensor size - since the Rebel, Rebel XT, 20D/30D, and 1D Mk II N have smaller sensors than a traditional 35mm frame (which is 36mm x 24mm), magnification is required so that traditional 35mm lenses can be used with the camera without an vignetting or other cropping. so, according to the sensor size, a certain degree of magnification is required (the Rebels and 20/30D are all 1.6x, the Mk II N is 1.3x - however, the 1Ds and 5D are both full frame sensors, so they will not require any magnification since they'll be using the entire lens area).
so a 10mm lens on a Rebel is actually 16mm in terms of the 35mm equivilent - if you took a 16mm lens and used it on a 35mm camera, the two would look the same.
but anyway, as far as your lens choice goes, get something affordable, and over 10mm. i've used some 8mm fisheyes and they're fun, but just waaay too distorted.