Not to resurrect an old thread, but in the interest of contributing information I used to ride with a couple different Mountainsmith lumbar packs. The problem I found for commuting is that they'd tend to sag down a bit in the back past the seat so when I'd slow down and move myself forward to put a foot on the ground, I'd catch the pack on the seat. Occasionally I'd fall over if I didn't lift myself up fast enough or it was hanging particularly low.

I love their packs, but they didn't work for me in my urban environment. I use a messenger bag now, thinking about a trunk bag.