"Even before Mr. Obama released secret memorandums on the interrogation techniques approved by the Bush administration, Mr. Cheney, as part of researching his memoir, had asked the National Archives to declassify two other documents he contends would show that harsh interrogations produced useful information...."
from Unemployed, Unapologetic and Unrestrained: It’s Cheney Unbound
I doubt it's going to happen, because Dick Darkside needs the aura of ultra-ultra secret "neo-con eyes only" intelligence lurking in the background to give him some credibility. If it were declassified (assuming it actually exists), chances are it would be discredited, as far as Cheney is concerned, in the subsequent episode of Countdown with Keith Olbermann, and there goes his credibility, assuming he has any left.
How CAN there be any truthful evidence of its effectiveness, with the abundance of evidence that it was never MEANT to be effective for purposes of gathering information?