Actually, National Bike to Work Day is in May, but I just bought a used bike this week off craigslist (a sweet hybrid for a good price) so today marked my first observance. I pretty much lived on my bike in college, but after having some bad bike karma in grad school I kind of fell out of the habit.
My route to work is pretty easy, about 2 miles mostly downhill, and faster than the bus or train. It's only the last few blocks that I get into the crazy downtown traffic -- dodging buses and taxis and whatnot. Plus my work has showers and bike racks in the office. It's a pretty sweet set-up so I think I'll probably bike in a few days a week and take the bus the rest of the time.
I've been running fairly frequently and think of myself as being in good shape, but I realized that biking definitely uses different muscles than most anything else. It's nice to use them again.