Cogito, ergo sum
View Fog is a simple method of adding fog to a map or an area in a map. It works by simply tinting the rendered scene by a specified amount. View Fog is sometimes also referred to as "fake fog". It uses practically no rendering resources, so it is very cheap resource-wise, but it is not that realistic. Often an underwater area will have a blueish view fog.
Setting Up View Fog in Unreal Tournament
Setting Up View Fog in UT2003
View Fog does not appear to work properly in UT2003, however, the variables for it are still present so theoretically it may still work on some graphics hardware. View Fog is controlled by PhysicsVolumes in UT2003. Specifically, PhysicsVolumes are used. To set up View Fog, simply set the PhysicsVolume->ViewFog property to whatever color you want the screen to be tinted.
Tarquin: so does it work or not?
SuperApe: I'm wondering if the author meant to say ViewFlash doesn't work in UT200x, not ViewFog. Distance fog seems to work fine. But the issue is confused by the properties ViewFog and FlashFog in DamageType. I found this page through a search for that stuff.