What default camera setting are you using? Check your pixel aspect ratio in Render Globals. If it's anything other than 1.0 your rendered image will appear distorted somehow.
The Standard NTSC 720x486 setting has a pixel aspect ratio of 0.9 This is necessary because TVs don't have square pixels, they have rectangular ones. Computer monitors however DO have square pixels.
Other settings have other Pixel Aspect Ratios.
Hope this helps.
-James