I'm using
the Canon backpack. It's about $65, and it holds a TON. I typically load it up with my XT with 400mm attached and four or five additional lenses, two filters, a wired remote, a lens pen, cards, batteries, a tripod collar, a panohead, and a tripod.
The downsides are that 1) you have to rip the Canon logo off if you don't want it to stick out as a photo backpack, 2) the removable straps on the bottom for securing a tripod are a pain, 3) it's not weather proof, and 4) it's not the greatest backpack in terms of comfort. But for the money, it hauls a
lot.