I have the Lowepro Stealth Reporter 200AW bag and normally the 70-200 2.8 IS is NOT in the bag due to the weight.
The 17-55 2.8 IS is on the camera, 10-22 and 50 1.8 as well as the 580EX along with some odds and ends are always in the bag. I'll trade the 50 for the 70-200 when there is a chance I will need it.
A lot of the time I will switch between the 17-55 and 70-200 for a family or indoor event.
If I am shooting outdoors and plan on long range shots I will use a monopod, put EVERYTHING in the back of the SUV and change between the 70-200, Bigma and the 10-22.
When using the monopod I still have the weight to carry around but don't have to support the weight of the lens while I am composing and shooting.
All that being said, I have had an XT till I sold it recently. The 40D comes tomorrow and I expect an increase in size and weight so things may change a bit but I think I will continue to use the monopod in a similar way.