As it's been said, the lens you need depends upon what you are pointing the camera at...
I recently purchased a new Canon 30d and a Canon 24-70 f.28 L to go with it as my general purpose lens, which suits my needs to shoot mostly faces (portraits, events, etc.).
I also wanted a fairly strong telephoto lens, but more for fun than work. Spending another two grand for my first choice lens (Canon 70-200 F2.8 IS w/2.0x tele-converter) was just not going to happen. In the end I spent $200 on a Sigma 70-300mm F4-5.6 after trying a number of similar lens from Canon, Tamron, Sigma, Quantaray, etc., I am very happy with this lens in terms of value. Value being the key word, for my hobby purposes, it's a nice lens with a nice case and a decent hood.
Moral of the story, the lens you need is very subjective. First figure out what you want it to be capable of, then determine your price range, then take your camera body to a few photo equipment stores and shoot some comparisons (go home, look at the images on a large screen, note what you like or dislike about each), then when you finally decide what you want, go price shopping online (just remember, bottom line price isn't everything so make sure your comfortable with the online merchants terms before you buy, all in all I've been very pleased with B&H photo for new equipment and KEH.com for used).
Best of luck, I love my 30d, I hope you have the same positive experiences.
David, A.K.A. Stupid