After opening the image in Photoshop I went to View>Show>Grid so that I would have guide lines for straightening out the bulging sides.

Then I started pushing and pulling the image to straighten it out.

Next I used the crop tool with the perspective box checked on the task bar to straighten out the sides, moving in the edges to line up with the angle of the distortion.

One final crop finished it, after which I resized down and neatened up the crop, adding a slight bit of sharpening.

If these directions aren't clear enough there are wonderful instruction videos for Photoshop on the web - just Google "Using the Warp Tool in Photoshop". Be careful that you don't go to a Puppet Warp Tool tutorial. That is an entirely different tool, only available in the latest version of Photoshop. (CS5)