Not sure what focal lengths you shoot at normally, or if you intend to upgrade to a full frame camera like the 5D or 1D series in the near future, but IMO the best walkaround lens suited to the 50D would be the EF-S 17-55 f/2.8 IS. It's sharp as, fast aperture, and IS. It's slightly more than the 17-40, so you'd have to weigh up whether you think the extra stop and IS is worth it. It's not an "L" series either, but the build quality is very good.
In addition, I find that if I want "wide" I usually go very wide and have the 10-22 also. It's a great lens, but in hindsight I'd have gone with the Sigma 10-20 probably (cheaper and just as good quality). Believe there is a new Sigma lens 10-20 or thereabouts being release soon that's f/3.5 constant aperture too.
If you consider 17mm wide enough, I'd go with the 17-55. Great combo with your 70-200 f/2.8L
