definately i recomend the basic 18-55 mm lense that is optional with the body, and dont worry too much about buying a super expensive telephoto just yet, but a 150-200$ usd. 300 mm telephoto is working quite well for me for the time being.
eventually, i hope to upgrade to an anti-shake telephoto, and even further, get a new telescope that i can attach the 20D with a t-mount for super hi quality telephoto views
but thats quite some time away, which is why i simply bought a cheap lense, as it works just fine for what im currently doing.
great camera, by the way