I went with a Lowepro sling last year and love it. It allows you to remove the camera w/o taking off the backpack. I hike a great deal and find the pack comfortable and the compartment sizes are flexible as they are velcro. I have the same problem you have with the larger lens. I would like to be able to to leave my 100 -400 on the body and have easy access to it, but as I have the 200 series there is not enough room. However the 300 series might work for you. http://www.onecall.com/ProductDetails.aspx?id=87578
The other drawback on this bag is that it does not have a built in tripod mount. I have made my own with some straps that work fine and still give me access to the compartment with out taking the bag all the way off. When you are walk somewhere and are not in a position to use a tripod, this feature is great. You just swing it around your body and the main compartmen is right there in front of you. It's especially nice in bad weather when you don't want to put the bag on the ground.
Price is around $100-$120
Canon XSi, Canon 18-55 IS, Tamron 28-300, Canon 100-400 IS L.