Good news! I just heard from Cat Allman at Google that they are willing to support Wikimania 2012 in DC at the diamond level (40k). The email I received was also sent to wikimania-sponsorship@wikimedia.org. It contains a purchase order number and instructions for billing. Will things just work from here, or do I need to do anything else?
A big thanks should go to Stefano Mazzocchi, Danny Berlin and Chris DiBona at Google who helped make this happen.
//Ed