It's not the carbonated beverage that affects beer, It's the soap used to
clean the glass. Think about those soap scum that builds up on the shower.
The same thing happens on a glass. When you pour beer into a glass that has
been washed with soap or some other detergent, it affects the head and
perception of the beer. For this reason, many places keep their beer glass
stock seperate and do not use soap to wash those glasses.
modnar