Decent camera in terms of Image quality, but the AF system is a far cry from the Pro level '1 series' cameras that the pro sports shooters use.
I'd recommend looking at the 'mid-level' cameras like the 50D or 7D. Either of these has pretty decent AF performance, which can be quite helpful when shooting a fast moving sport.
Something else to consider, is the burst rate. (how fast the camera can shoot consecutive shots). I believe the Rebel cameras are around 3 frames per second, while the 50D and 7D are more like 6 or 7 frames per second.
You really need to consider lenses. You may want a telephotos lens, especially when shooting something like football, where you can't be right close to all the action. You also want a 'fast' lens (large maximum aperture) which allows you to get faster shutter speeds which are required for getting sharp shots. The problem is that fast telephoto lenses (like pro sports shooter use), usually start at around $5000 and go up from there.
There are obviously cheaper options and compromises to be made, but it all comes down to how much you have to spend, the conditions you will be shooting in and your expectations for the photos.
You don't have to spend a lot to get some snaps of your kid playing football...but if you want photos that look anything like what you see in S.I....then you may need some heavy gear.