perspective correction similar to PSCS2 Lens correction.
Placed a circle in the center of image for illustration.
set guides on each side of circle to use for shape adjustment and 2 guides
near the edges for perspective alignment.
First stretch the top and compress the bottom until the building lines are vertical,
while keeping the center side handles aligned with the edge of the image.
Then adjust the top and bottom center handles until the distorted circle
is reshaped to fill the guide outline.

forum thread: http://forums.dpreview.com/forums/read.asp?forum=1006&message=17494422