It is something that has some fact, some fiction to it.
Many cytics taste like salt. It is a byproduct of CF that you excrete excess salt in your sweat. This is the basis for the 'sweat test', it merely makes you sweat and measures how much salt comes out.
So, in many cystics, you can actually taste the salt when you lick them. But it it not 100% by any means.
FYI, yes I have licked my son, tasted like a day-old potato chip <img src="i/expressions/face-icon-small-smile.gif" border="0">