I came across an excellent must-read post for anyone who thinks they want to do freemium, written by John Greathouse, who was involved in the precursor (?) to GoToWebinar (BuddyHelp). I can’t find much info on BuddyHelp… but hey, that was internet eons ago.
Anyway, John writes How To Make Freemium Customers Generate Revenue For Your Startup. Go read it, as he seems to have identified a lot of pitfalls that you can avoid.