This kind of functionality is actually available out-of-the-box as Membership sites, where users on their MySite can see what sites they are a member of. SharePoint maintains a list of membership sites for each user - that is a list of all sites where the user is granted access through the SharePoint Member group on sites.
See My Sites overview on MSDN and this related question.
Granted, the Membership sites functionality in SP2007 and SP2010 is not very flexible. It only works with the default Member group on sites. Permissions granted through other groups are ignored.
In SP2013 all this changes as users will simply be able to follow the sites they care about.