This isn't a recommendation for a laptop bag but another alternative to carrying one. This obviously depends on your setup - if you already have a computer at home that has the same software as the one at work (or leave the laptop at work), etc etc and if you don't absolutely have to have the laptop with you the days you take your bike in.
I have a workstation at home and at work. I work with large relational databases in the GB range. I have a wallet size Maxtor 80 GB hard drive (that cost less than a 4GB mem stick, for some reason). I copy huge directories off the server at work with the latest updates in about 2 minutes with the USB port at the end of the day. Bring the hard drive home and transfer those server directories to the computer at home and I can work. Actually, I got a free synchronization software off the web and I just sync the directories whether I am at home or work. It works out great, and I just slip the hard drive into my Trek Interchange pack on the bike and off I go.
A guy at work who rides a motorcycle also does this.