Unfortunately this is one of the side effects of having an increased field of view. Much like a fish eye lens on a camera, the wider the FOV the greater the distortions.
The only way around this really is to reduce your horizontal FOV which unfortunately reduces your vertical FOV at the same time in a lot of older games (HOR-), so for some games it just makes them feel too claustrophobic if you loose too much vertical FOV. The trade off is that it can reduce the pincushion effect but you arent then really getting the full benefit of the wider view and it often feels uncomfortable. Most games these days are HOR+ which means vertical FOV is preserved even if you increase your resolution by using triple screen resolution. The side effect of this is often more exaggerated pincushion. Some games though have distortion even on a standard 1920x1080 display because of the default FOV, this is by design to make them more comfortable to watch. (watch the video below for an explanation)
The other factor that effects this is camera perspective which also interacts with FOV. Most games are written with a fixed camera perspective. That is... if you change the FOV you cannot change the camera persective (position) in order to reduce or remove the distortion.
How focal length affects perspective: Varying focal lengths at identical field size achieved by different camera-subject distances. Notice that the shorter the focal length and the larger the angle of view, perspective distortion and size differences increase.
taken from http://en.wikipedia.org/wiki/Angle_of_view
Also check these out! :)
http://en.wikipedia.org/wiki/Field_of_view_in_video_games
Cool video tutorial on Game FOV >>> http://artsygamer.com/fov-in-games/
http://unity3d.com/support/documentation/Components/class-Camera.html