Just received the following e-mail from US:

"Double the fun when you go to Rio
Celebrating our new service with double Preferred-qualifying miles*

Make it a double. Travel to the Marvelous City with US and earn double Preferred-qualifying miles.

Register now
Book now through September 30, 2009
Travel between December 2, 2009 and February 28, 2010"

The link is here.

It looks like you have to register in advance.