I'll give you my answer as a total newbie to dslr's. I got my 30D in December.
I got the 28-135 with the camera...nice lens and i took some decent pictures. I found that most of the pictures I took are inside, family pictures, so low ight and up close. I found the 24 was not wide enough.
I am about to sell it.
I bought a new tamron 17-50mm f2.8 and really like it, but I have found that my hands are not as steady as I thought, so now I wish I had sold the 28-135 first (or not bought it at all), and put the money towards a 17-55mm IS, which would be nice for low light indoor pics.
My original thought was "Walk around lens", then realized as I live in Canada, there's alot of indoor time spent in our long winters, so the 17-50/55 f/2.8 is a better option for me.
I also bought a used 50mm f1.8, but didn't find any real use for it, so it'll be sold. I also bought used, a 10-22mm and a 70-30mm IS, those lenses make me want to go outside and take pictures.
I'm going to Greece and Turkey in the fall, and I'll bring all 3 lenses (10-22, 17-50, 70-300 to see what I use for vactions.
So to answer your question, you need to really think about what pictures you are going to take the most in the near future, then add on to your collection as the need (want?) arises.
I know I've wasted some money on the 28-135, when I sell it, and maybe the Tamron is I go to the canon version, but I've learned what I need to take the type of pictures I frequently take.
Now if they only made a 17-150-ish f2.8 IS lens for a reasonable price with great IQ, all us amateurs wouldn't have to by multiple lenses
