I have heard the actual screens are actually a protective "screen protector" in themselves and are very easy to replace (Canon makes a little kit). The cost isn't that much more than an aftermarket screen protector. I just bought a 50d, and I'm going without a protector. The screens are made to take normal abuse, and my 20D never got a noticeable scratch.
Now, my backpacking GPS, on the other hand, isn't as forgiving, and I have a nice little scratch on its screen.