I've been playing around with the beta of Microsoft Forefront Threat Management Gateway (TMG for short) since its release in late spring.
Last week, I was looking to do some testing, and logged on to the TMG box. I noticed that the 'Update Services License' had expired. (When the TMG Beta is installed, a 90-day license for Malware Inspection is installed)
Well, lab environment or not, such details have to be taken care of, so I rummaged around a bit in the interface. Turns out that addressing this is really simple:
Right-click the 'Malware Inspection' service entry, click "License Details", and change the expiration date to your liking. Your TMG Beta should now be able to download updated malware definitions again.