For about a year, I had a 30D + 10-22mm as my primary WA lens. I also had the thrifty fifty (nifty fifty, your pick) and a 28-105. Looking back, kind of a waste, lol.
In any case, my thinking was different back then, or lack there-of. I now have a 40D+17-40L as my primary. Actually, it IS MY ONLY lens. I shoot primarily landscapes. Only on several occasions do I wish i still had my sigma 10-20 (oh yeah, i had an XSI + Sigma 10-20 and EF-S 55-250). But that is very seldom do i wish that. I think the 17-40L is really a nice combo. Sure there are people that are going to say 17-55. I am sure it is a great lens. If I was to get 2 more lenses, i would get the 24-70 2.8 and probably either the 100-400 or the 70-200 2.8.
My best advice is to check PBASE, photosig, smugmug and see the results of the lens/body combo. You already have a lens that pretty much covers the wide aspect. Can you live with that? The 17-55 covers the same range, and the 17-40L will be shorter. If you need fast, i would go with 17-55 (would probably get a 24-70L though). If not, 17-40L is an amazing lens.
GO to a camera shop and check out the lenses. TO see the 17-40L and XSI in action, click on either link in my sig.