Hmm, I don't think anyone has ever made a study of it...sooo...TAG! You're It!

Try http://www.weddslist.com/cgi-bin/tar...ce=misc#result
http://houseoftartan.co.uk/interacti...ver/index.html

Alternatively, you can buy a cd-rom based generator here:
http://www.tartangenerator.com/