A rather toungue-in-cheek activity at geek-fest conference Extreme Computing 2002 had this description:

"Your supermarket loyalty card - it just sits there in your purse or
    wallet, silently recording everything you buy, occasionally rewarding
    you with just enough points and prizes to keep you taking it to the
    shops. But now it's your chance to get your own back - at EXTREME
    COMPUTING 2002, where you can swap your loyalty card with someone
    else's (cash in the points first if you must), and take on the brand
    new demographic profile of a complete stranger."

Er, yeeees. I hope they were aware that the demographic profiles would all amount to the same thing at such an event: young, single males buying convenience ready-made-meals-for-one.