I've been marking rides to and from work as commutes pretty diligently, but I see from various reports on the site that commutes can be hidden, which makes me think maybe they aren't meant to be treated as significant exercise? I ride as hard on commutes as I do on non-commutes, just that my destination is work and I have paniers on my bike for a change of clothes, laptop, etc. If anything, that little extra weight burns a few more calories, I'm sure.
Would others count these as commutes? It is a little neat seeing how much CO2 I saved by not driving, though that's not really why I do it.