You have a 70-200/2.8 IS and you're happy with the 17-85? AS I said, you are a very interesting person.
The 17-85 is very sharp at the long end when stopped down to f/8 or f/11, but it's a bit soft at the wide end, but i rather have a bit soft image than no image at all. On a 20D the 17-85 is a very good walkabout lens for me, 24 is not wide enough for me.
I alredy have primes in 24 (f/2.
and 100(f/2) that does the job the 24-105 could do when I need better quality than the 17-85, so for me a new zoom in that range would have to be faster that f/4 and must have IS to get my interest, in addition to top optical quality. If I buy a FF camera (maybe Canon comes with somthing I like in february?) I will probably get the 24-105 as a walkabout lens.