As others have said, B&N uses the CC# in their DRM. However, most ebook stores require a CC# on file because they use this in their geographic restrictions enforcement. What Amazon does instead is snoop on which country you appear to be buying ebooks from (i.e. where the purchasing computer or Kindle is located). Some other stores do this too, but Amazon uses it as its main approach to conform to the geographic restrictions imposed by publishers.
EDIT: A recent post to his blog by
Mike Cane indicates that he can buy Adobe ePubs at Sony’s Reader Store, the Kobo Books store, and Borders without a CC. These would all work on your Nook. He also indicates that he has an account at Amazon without a CC, so it may be possible to buy Kindle gift ebooks and redeem them yourself without a 1-click CC registration.