I'm an American, and I've always preferred the word diapers. I don't think I could get used to calling them anything else.
An interesting note about the word nappies-- the French use the word "une couche". In French, the verb "couche" ("coucher," actually) means to go to bed, in other words to take a nap.
So...both UK and French versions involve taking a nap or nappies or what have you. I don't know any other languages, but it would be interesting to know how the words are related, if anyone knows.
I just thought it was kind of cool.
Diapers for life!