I've seen a rash of reports lately with people having problems with TFS when configured to use a Fully Qualified Domain Name (FQDN).  There are certain extranet scenarios when you need to configure TFS this way.  The problem has been isolated to a change in Windows 2003 SP1.

The symptom is error messages that look like the following in your event log:

Exception Message: TF30063: You are not authorized to access https://teamsystem.contoso.co.uk:8081/. (type TeamFoundationServerUnauthorizedException)

Read the following KB article to understand the issue and learn how to correct it: http://support.microsoft.com/kb/926642