I wish canon made a 17-300mm f/1.2L IS USM. I would never have to take it off my camera. But then again the thing would probably weigh 20 pounds and cost $10k
I picked up a Sony a100 with the kit lenses 18-70 and 70-300. Then with in 2 weeks I picked up a 50 f1.7. I find my self using the prime more than the small zoom lens. I am actually contemplating getting rid of the 18-70 and getting another, shorter prime.
My opinion is, if you have a sigma 28-70 2.8, just but the 40D and start shooting, and save up some cash for future glass, or if you want to spent your full $1500, skip the 1.8 and go for the 50mm 1.4.