The Ninth Circuit recently issued an opinion in Yellow Cab Co. of Sacramento v. Yellow Cab of Elk Grove, Inc., No. 03-16218 (9th Cir. Aug. 9, 2005), in which it re-affirmed that the owner of an unregistered mark confronted with a challenge to the validity of the mark (such as the genericism defense raised in the case) has the burden to prove that the mark is valid and protectable.
The court performed its genericism analysis by literally asking itself and answering itself the questions "who are you; what are you?" I'm not sure I've ever seen genericism analyzed in a way that sounds like a game kids play at preschool, but, hey, whatever works.