I believe 100 watt bulbs have been outlawed, at least in California. I know my local grocery store has 95 watt bulbs and I'm seriously thinking of grabbing some just in case I need them later.
You are probably right, summoner, about the efficiency, but ease of acquiring is definitely a factor. With gas here at just about $4 a gallon the corner supermarket is already $1 cheaper than the closest Home Depot. Add to that the inconvenience of going to Home Depot in general and the supermarket seems very attractive indeed.
Of course, if I had many chameleons I might feel differently.