This promotion was published on United.com few days ago, then they removed it because they decided not to do it. However, You can still register it by call United Airline Mileage plus customer Service, not reservation line.
Mention promotion code MPD698,
Offer valid for booking and travel btw Nov 13th to Jan 31st,2009 with blackout on Jan 2-5 and Dec 20th.
More discussion on this topic here