Thanks to all who shared their advice on the right bag to use for my laptop and D-SLR camera/lenses/accessories. I ended up selecting the Lowepro CompuDaypack. I took it with me to Washington, DC this week and it fit all my gear, if a bit snugly, and was able to be stowed under the seat in front of me on the airplane (again, a bit snugly).
Overall, I’m pleased with it so far. It held my Canon EOS 30D, Speedlite, 3 lenses (18-55, 28-200, 50), and various computer and camera accessories. I also managed to fit my MacBook Pro with no difficulties and squeezed in an umbrella, various cables, a small Sony Handycam, and a few other things.
At this point, the bag is at its limits for me, so if I ever end up pulling the trigger to get the 70-200 IS/2.8 Canon L lens I am lusting after, I’ll probably have to get something bigger and live with stowing it in the overhead bin.