the keystone effect" and it can be very annoying. But there's a quick way to fix it in Photoshop, so your snapshots look more professional.
In CS5, just go to Filter-->Lens Correction
A pop-up window will now appear with your image at the left and some distort options at the right. The pictures are pretty self-explanatory. Go to the vertical and horizontal perspective sliders and drag to the left or to the right (go ahead, be extreme at first so you can see what it's doing—or you slide them like crazy and pretend you're in "Inception") until you fix the perspective.
One more difference between CS5 and earlier versions. As you shift the perspective, CS5 crops as needed. In earlier versions, once you commit your transform, you will have to re-crop your image. Toggle the background layer off and you will see what I mean. If your initial image it too tightly cropped then you may end up losing parts of the picture you wanted to keep.
There you have it. A super quick fix for a very common problem.