Anyone know of an existing utility that will display a list of the downloaded hi-res dds files under \Data\Textures, and also display their image?

I'm really interested in building my own version to help sort out and track which textures are hi-res, and which ones are from the base bsa's.

I am not sure about listing dds files, but but at least WTV can display them and is relatively handy. It is pretty old but has been created specifically for that purpose. No cube maps, though. Perhaps it might not exactly suit your needs, but you can always give it a try. It is available from nVidia http://developer.nvidia.com/content/legacy-texture-tools or http://www.nvidia.com/object/windows_texture_viewer.html.