DaveJ's site and Half-Life ERC have updated their sites with info about the new command in 1104, gl_wireframe, which allows mappers to enable the entire PVS (potentially visible set) to be outlined. This will show you exactly what the engine sees and how it is broken into polygons. This should be an essential tool that all mappers should use. For more info and some screen shots of gl_wireframe in action, check the sites out.