Well, look what I found in the Exchange 2010 document site - Using Threat Management Gateway With Exchange 2010 which you can find at http://technet.microsoft.com/en-us/library/aa998036(EXCHG.140).aspx
Unfortunately, there really isn’t any information on TMG at this site (yet). There is a note regarding this entry that says: this topic’s current status is: Writing Not Started.
I say it’s time for us to get the writing started! We’ll see if we can get Exchange 2010 working in our lab and see if we can get TMG to publish Exchange 2010. Then we’ll see if we can get IAG to do the same thing.
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer