There is a difference between stretching and skewing though. A game that isn't hor+ won't fill up the added space with game world it will just stretch the normal view out. Most games aren't like this anymore but I know Heroes of Might and Magic 5 was. A vert- game will actually remove viewable area vertically to cut the viewport down to 48:9 aspect ratio, effectively letterboxing out the sides without stretching. Every UE3 based game (Mass Effect, Borderlands) is vert- past 1920x1080 and requires some FoV tweaking to fix unless the devs fixed it themselves (Bioshock, Batman:AA). A game that is hor+ actually renders more game world to fill up the area, this is best, BFBC2 is a perfect example.
Skewing happens because of the fish eye effect devs use to give you a sense of peripheral vision on single monitor so you don't feel claustrophobic. Some games have really strong fish eye lens and some don't. Fish eye always causes skewing toward the edges of the viewport, at ultra-wide resolutions this becomes much stronger because the skewing increases further from the center point..
Like Lonyo linked widescreengamingforum.com has a ton of info on this. I'd take a quick look at the 'surround comparisons' at this link
http://www.widescreengamingforum.com/screenshots
Here is an example from F.E.A.R. You can see the sides start to skew from the fisheye at triple monitor, but there's also a dude over there shooting you.