I also recommend checking out the 17-40/4L. I think it is the closest thing available to what the original poster asked for in terms of range, quality, and price.
The build quality's there, and the range is sort-of close, but I am not buying/carring around an f/4 normal lens. The IQ of the 17-40 is also fairly poor at 40mm, which is where it counts (moreso than at the wide end, where you don't tend to crop, or at least don't plan to).
Basically my checklist is as follows: (hope I'm not leaving anything out)
- L optical/build/AF/etc. quality. Weather sealing would be nice, and I doubt that Canon would cheap out on that. At these focal lengths, I also expect that they can build something on par with the 24-70 at the very least, give or take a tiny bit at the wide end.
- 20mm through 50mm at least (I don't care about anything outside of this, and a 2.5x zoom doesn't represent a lot of engineering compromises). I can't live with just 24mm, I have no use for anything under 20mm, and I don't see the point in changing lenses to have a 50mm given all of that.
- f/2.8 -- I don't care as much at the wide end, but I don't want a 50mm lens that isn't usefully fast under most conditions.
I have, as far as I can tell, totally done my homework on this, and concluded that such a lens a) would probably be useful for a lot of people and b) doesn't exist. I could probably list 95% of Canon's L lenses and non-L primes off the top of my head, so there's nothing I haven't already considered. I've also verified that Sigma/Tamron/Tokina don't offer anything remotely close (except for the Sigma 20-40/2.8, which is garbage).