For any of you 660 owners that are thinking of buying the 665 for the new features it is being rumored/leaked that everything the 665 will have will be available on the 660 (see above in red). With them both using the same cradle makes me think that a firmware upgrade may give the 660 owners all the new features..... time will tell. 

Ride Safe,
Fired00d

Here is the response from Garmin Support when I asked this very question....
Subject:
Zumo 660 Upgrade
Message Body:
Will the Zumo 660 be upgradeable to XM Radio/NavWeather capable. I am going
to cancel my 660 order (bought with new Harley Davidson Motorcycle) and
wait for the 665 to be available through Harley Davidson unless I get
confirmation that the 660 will be able to support XM with a future
upgrade. Thanks
Dear Carl,
Thank you for contacting Garmin International.
At this time, the Zumo 660 is not going to be compatible with the XM
antenna. Please let us know if you have further questions.
With Best Regards,
Shawn K
Product Support Specialist
2nd Shift Automotive Team
Garmin International
913-397-8200
800-800-1020
913-440-8280 (fax) Att: Shawn K, Associate #6144
www.garmin.com