If the contents of the UserInfo table(s) has become out of sync with the Profile database for a given user and the Profile Sync job doesn’t pick this up the approach is usually to remove the user from the All People list for the Site Collection(s) and re-add them.
However, what if you’re using AD groups and the user doesn’t appear in the list?
Simple! Add them (to and group – e.g. Visitors) and then remove them. You should then find that the content db for the Site Collection has been updated.
UPDATE: See here http://msfarmer.blogspot.com/2009/11/user-profiles-changes-are-not-updating.html for an explaination of the relationship between this behaviour and WSS users being ‘inactive’.