I have a 300D, and my friend, who is a wedding photoagrapher, has a 20D and a5D. He's teaching me how to use the 20D so I can help him out at an upcoming wedding.
All of the points Kelley made are valid, but come more from the perspective of a pro or serious ameture. My observations come more from the perspective of a moderate to serious ameture, much lower on the photographic food chain, as it were.
1. The 20D has a much sturdier body than the Rebel. It has more metal in it, and is larger and more comfy in the hands of someone with big hands. However, it's also heavier, and might not be so comfy if you have small hands.
2. The 20D has more features, more controls, and more capabilities than the Rebel. This makes it much more suited to high-end photography, but also makes it much more complicated for simpler stuff like vacation or family photos.
The 20D is a better camera in almost every respect, but if you're just going to be using it for vacation pics at WDW, family shots at the holidays, kid pics at soccer games and school functions, you could get away with a Digital Rebel instead. Most importantly, if the size and weight of the camera are a serious issue for you, the Rebel is smaller, lighter, and easier to handle for small hands.
Your Milage May Vary. I chose a Digital Rebel 300D mostly because I could not afford a 20D, but also because the 20D would have been a bit of overkill for the casual type of photography that I do. If I were more serious about it, I might consider stepping up to the 20D, and believe me, it is definitely a step up.
I don't think you'll be dissapointed in either camera.
But one thing - budget for a lens. The Rebel comes with a very usable 18-55 lens, but 55mm is just not enough zoom for WDW, and certainly wouldn't be enough for a sporting event or school function. I just ordered a Sigma 18-125 lens which will give me roughly 6.9X zoom, compared with the 2.8X or so that I get with the 18-55 lens that came with my Rebel.