For the (semi) Pro Photoshopper
Here is a link to a Youtube video that shows how to use the crop tool to correct perspective. http://www.youtube.com/watch?v=xKXEr0dZwM4&feature=player_embedded
While the video shows the tool on a single image, it is not hard to see how the results could be used to replace an off-plane image. This is definitely not your usual Photoshop ‘tip’ tutorial.