Dopefish, based on your description, I belive what happens is that horizontal FOV becomes greater or equal to 180 degrees. This doesn't work in 3D graphics because at 180 you get near and far plane parallel with frustum sides and angles above that result in something like reversed projection.
This puts a limit to how wide your multimonitor setup can be in order to preserve the vertical FOV similar to single screen. For example, with three monitors 4:3 (total aspect ratio 4.0) and with maximum horizontal FOV of 180 degrees (actually, it has to be something like 179.9 to avoid the aforesaid problem) you can implement a perfect hor+ only for those games whose vertical FOV is less than 45 degrees.
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