I finally got some straight answers from Toyota....the present Navigation disk available for the Prius is version 5.1. My 2005 has version 4.2. In October of 2006 version 6 is due to be released. I am going to wait until October to upgrade mine since October is right around the corner. The cost was going to be approximately $220. So, if you are wondering, that is the deal.
Thank you for the information. I guess everyone will have to judge how important the upgrade was to them.
I have no idea about the application to the '06 model - but I would think it would work on the '04 and '05. As far as I know it is only the mapping portion and no other changes.