You need to check the Personalization Service Permissions on your Shared
Services Provider. This is where the permissions are set for who can
and who cannot see the MySite link. The default is to have
NTAuthority/Authenticated users have Personal Features and Personal
Sites permissions. These permissions are what govern whether you see a
MySite link or not. If users aren't authenticating via AD, if they are
accessing the site anonymously, or if these rights have been changed
from the default setting then they won't see the MySite link.
I tested this be creating a user and giving them ReadOnly access to a
SharePoint MOSS site. They still have the MySite link. If I remove the
Personalization Service Permission refrerence above the link goes away.