On my Lucid Lynx, I can create a share with Nautilus, but nobody can log in from outside, because there is no user/password defined for that share. That's definitively wrong. Fortunately, it's quite easy to create a user:
sudo smbpasswd -a <your user name>
If you forgot to create the smbpasswd, your files might be owned by nobody:nobody. Bestow ownership recursively with (cd into the shared folder first, or repace the dot with the path)
find . -type f -exec chown user:group {} ;
find . -type d -exec chown user:group {} ;
If you need to recursively set file permissions, use:
find . -type f -exec chmod perms {} ;
find . -type d -exec chmod perms {} ;