Here in Maine, we have an "commuter connections" organization promoting alternatives to single-occupancy commuting, and every year, they sponsor "Commute Another Way Day". Bicycling is mentioned as one alternative, but public transportation and carpooling are also promoted. Today is Commute Another Way Day for 2005. In addition to 2 of us who regularly bike commute anyway, and one who often walks, we had 5 car-poolers and a bus rider for the day, and we all had a "pot-luck" breakfast in the kitchen.
I know most of you aren't in Maine, but I mention it just for your awareness. If you cycle partly due to environmental or oil usage awareness, I think it's an even better idea than just "bike to work" days, since it promotes all the alternatives on a single day.