There's a 12% increase in pixel count going from 1280x720 to 960x1080 (although the memory increase is negligible). I don't really know how this translates into actual, real-world performance penalties.
I actually asked a developer from Monolith about this recently, about why their upcoming games don't support 1080 upscaling. He said that he looked into this functionality, but decided that the performance decrease was too great. He claimed that the 12% increase in pixel density was big for a shader-intensive game like theb upcoming Condemned 2.
I don't know how much credence to give to this statement. Is he saying that titles like Uncharted, or Unreal Tournament III, aren't shader-intensive?