Is this the sort of hat you mean Loz?
http://www.villagehatshop.com/propeller_round. html
I believe they are known as propeller beanies, and seem to have been adopted by those who regard themselves as "nerds / geeks" as a kind of mark of belonging, or badge of honour. I have no idea as to where the association originated - I would imagine there is probably a film involved somewhere down the line - one of those 1980's american college campus comedies perhaps?
Any explanation of this minor phenomenon would probably involve words such as "ironic statement" or possibly "post-modern expression"