I think LATS has hit right on. Not acknowledging Spirit as the original composers and paying royalties would be like Stevie Ray Vaughan blatantly taking sole credit for composing and originating Little Wing or Voodoochild, which was originally done by Jimi Hendrix.

Obviously its not a matter of which version is more widely acclaimed or what not it is giving the originator there rightful credit. This is nothing new bands have been covering and borrowing rights to songs for as long as copyrighting has been a part of the music business.

Also I am a HUGE ZEPPELIN fan and am NOT a pessimist so don't pin that stereotype on me! I am just a firm believer in giving credit where credit is due and abiding by laws that are set in place with no if ands are buts about it! Ok I'm through :)