They must have done a later one because I recall in one of the relatively recent Dragon's, they commented about how the readership got younger, and a smidgeon change in gender percentages.
But, I cannot remember when the did it.
Also, they had a bit a while ago where you could win a bunch of Paizo stuff if you answered their survey.
/edit after grabbing a back issue I happened to just be reading today:
IN July 2006 they were advertising for a Dungeon And Dragon reader survey, where you could win a bunch of Paizo stuff. The linkie in the ad is paizo.com/survey The survey ran from April 21 to August 18, 2006