Well, it turns out that YES it does have to be soldered. It's a mechanical issue though. If you don't solder it, then the pad solder joints will end up being cracked by any small amount of board flex that might occur in thermal cycling, or a drop. Many thanks to atmel for failing to put this information into the data sheets. It is circulating within atmel, in the form of a report attached to an email.
Message
Re: T-26 MLF package
2005-11-09 by Dave VanHorn