I have the Lowepro Mini Trekker. It comfortably holds all the camera gear and also the T pieces and bits and pieces for astro work as well. My camera tripod clips on the outside of the pack and makes for a nice neat package.
When we go bushwalking my wife carries that with all the camera gear in and I have a day pack with the wet weather gear, first aid kit, water and the kitchen sink
in. The obvious benefit of doing it this way is I choose the lenses (and no she doesn't visit the forums [I hope]).
The Lowepro gear is not cheap but the quality is first rate. I bought myn at Camera House and along with a spare battery for the 400D I got a reasonable price negotiated.