Don't you want "guest ok = yes". Also check the permissions on the directory being shared.
The "Samba 3 By Example" book has a similar setup and includes instructions on how to create the drive being shared. They created a user and group for the share and used "force user" and "force group" instead of using guest. You may have a samba-doc package that contains either a ps or pdf version of the book, or you can go to the samba.org website and download it.