Did a search and didn't see this posted. Found this on HHonors Twitter feed.
http://hhonors.com/VisaSignature
Instant Gold Membership if your card says Visa Signature
Enjoy Hilton HHonors Gold tratment at any hotel or resort in the Hilton Worldwide portfolio for stays before 8/31/11. Stay 3 times by 8/31/11 to extend your Gold membership through March 2013.
I entered the full CC # of my AA Citi Visa Signature and also my HHonors number. My HHonors account was upgraded to Gold immediately!
ETA: Seems like you get 5,000 bonus points if you are already an HHonors Gold or Diamond elite member.
ETA2: Some Gold members are not getting the message about the 5,000 bonus points. So YMMV if you are already Gold.