The Gentoo 2005.1 Handbook, chapter 5, item 5.d indicates the wrong directory for the portage snapshot dir, in the document it says '/mnt/cdrom/snapshots' but this directory actually doen't exists. The right directory would be '/mnt/cdrom/snapshot'. The iso image i used it's the universal installation, version 2005.1.
It was been fixed in 2005.1-r1. Please look at http://bugs.gentoo.org/show_bug.cgi?id=102128 Thanks for reporting anyway.
Sorry, i tried to find bugs reporting this error, but couldn't find any. I better learn to do a better search, anyway. :] Regards.