I take our pup to "doggy day care" twice a week. She gets to run and play and it really tires her out. I haul her in my daughter's old Trek trailer, which has a flat bottom with a foam pad. I then go to work with the trailer. It's been handy a few times, as I was able to stop at the store on the way home or haul extra items to and from work. Wouldn't want to haul it every day, though.
See http://www.bikeforums.net/utility-cy...l#post16819278