By their very nature, lenses add distortion to the images they capture. I've posted a new tutorial over at TipSquirrel.com that addressed how to fix lens distortion for both stills and video.
“One of the commands in Photoshop CS5 that received a major makeover is the Lens Correction command. It’s no longer buried, rather it sits proudly near the top of the filter menu. Adobe now ties the filter to specific lenses in the file’s metadata. You can even create your own lens profiles with a great utility. Visit http://labs.adobe.com/technologies/lensprofile_creator/ for full details.”
Check out the full tutorial here for free.