I can't find anything about having to pay with the Visa Signature for the stays in the T&C on the sign-up page:
"Hilton HHonors Member must register by August 31, 2011 with their valid U.S.-issued Visa Signature card at www.hhonors.com/visasignature. Hilton HHonors Members with Blue or Silver status or Gold members not currently participating in another fast track offer will receive HHonors Gold Elite status at the time of registration valid through August 31, 2011. By completing three qualifying stays at any participating hotel and resort in the Hilton Worldwide portfolio by August 31, 2011, Member will retain HHonors Gold Elite status through March 31, 2013. Thereafter, standard requirements must be met to maintain HHonors Gold Elite status. If Member does not complete three qualifying stays prior to August 31, 2011, Member will return to their former HHonors tier status after August 31, 2011, and standard tier requirements again apply. Offer available only to valid U.S.-issued Visa Signature cardholders and is not transferable. Full details of HHonors Gold Elite Membership will be mailed to Member after completing three qualified stays prior to August 31, 2011, or click here to HHonors Gold Elite benefits are subject to availability and may vary at each hotel. This offer may be terminated without notice.
Hilton HHonors™ membership, earning of Points & Miles™, and redemption of Points are subject to HHonors Terms and Conditions.
©2011 Hilton Worldwide"