Walk through most airports en-route from check-in to the plane and see how many PCs you pass which are running XP - I did it yesterday and my count was 4 (at least 2 of which were sat beside dot-matrix printers, which probably also says something about "if it ain't broke don't fix it").
I travel most weeks, and the count at most airports is usually about the same, which when you consider how many gates most airports have these days quickly tots up to a lot of XP installs...