Well after some failed attempts to pan at a mountain bike race a couple weeks ago with my Sigma 70-200, I gave it another shot yesterday using my 50mm 1.8 prime lens. I found that my panning was much improved. I think the 70-200 is just too big and front heavy to pan on the Digital Rebel without a battery pack to offset the weight.
However, the lightweight 50mm seemed to be a great lens for getting pan shots for me. I had about a 50 - 60% success rate. I also found that shooting continuously seemed to help, even if you hit all 3 or 4 shots you can pick the best.
Anyway, here is one of the pics I got from the weekend. This is the only one I have access to here at work, I'll upload some of the other good ones tonight. Thanks to all who helped me get things dialed in on the pans.
I used ISO100, 1/40 shutter speed in TV mode with no flash since I was in the open. Also, I would use Manual Focus and focus in on the ground where I expected the rider to come through, eliminating the chance I wouldn't be able to get/keep a focus lock.