Since Team Foundation Server 2008 was released end of November 2007 the early adopters which for some reasons have not obtained a product key yet are facing the problem of expiring trial licenses on their TFS servers.
I definitely recommend to find out when your TFS trial expires. You can use the Version Detector Tool , which Brian Harry  blogged about.
If you have trouble getting a product key in time:
- From ten days before it expires you can extend the trial period for 30 additional days (using the tool mentioned above).
- If that’s not enough contact Microsoft to get additional 30 days.
Eventually people will check for the expiration date before they actually learn it the “hard way”…
So: don’t be a fool, use the tool!