Suzie, I use a backpack to commute with. It's actually not that bad, you just have to make sure it rides up high on your back (i.e. crank those shoulder straps tight). As long as I'm not a packrat and only bring the minimal stuff (small camp towel, travel shower stuff, clothes for the day/LIGHT shoes... this requires serious outfit consideration but is easier in the summer) then it's not bad, and I ride a road bike and even find it comfortable to ride in the drops with the bag on for most of the time!

I would love to get some of my coworkers to ride in, but they've got the kids excuse. I'm working on them for the summer though...

K.