I just signed up for TGM a couple of weeks ago and my understanding is that you can sign up any time in advance of your trip. Your subscription will expire 60 days after your visit (he has you enter your visit dates).
I have found it very helpful. Even though the December 2006 information isn't out yet (we are going 12/19-12/22/06), the archived info is great and I think I have learned enough "little tips" to make it worthwhile!