If you us Microsoft Office SharePoint Server 2007 with your TF server and have applied the recently released Service Pack 2 to it, chances are that the MOSS server now thinks it is a 180 day trial version, and in 180 days client access will be denied. This issue is caused by the MOSS 2007 SP2. Some key points:

• Products that will have this issue after applying SP2 include MOSS 2007, Project Server 2007, Form Server 2007, Search Server 2008 and Search Server 2008 Express.
  
• WSS 3.0 is not affected by this issue
  
• Even if your MOSS install is made to become a trial version by installing SP2, and this expiration date is hit 180 days later, your data will be safe.  Product expiration 180 days after SP2 installation will not affect data, configuration or application code, but will render SharePoint inaccessible for end-users.
  
• At time of writing there is a HOTFIX in the works to mitigate this issue. In the meantime there is a manual process available to resolve it.
  

More details on this issue and how to fix it are available at the SharePoint Team Blog here:

http://blogs.msdn.com/sharepoint/archive/2009/05/21/attention-important-information-on-service-pack-2.aspx

HTH,
- Trev