Umm , RobsTV , have you actually tried this yet with verifiable results ? Cause Twinview is NOT a feature of the GTS. It's specific to the MX. Wouldn't make much sense not to advertise it if it's on the GTS's too. With the Ati All-inWonder I had, the Tv-out displayed EXACTLY what was on the monitor on the Tv-out. The ability to do 2 separate signals is the exception, not the norm. I have also done the same experiment with a TNT1, a TNT2, and a voodoo rush. As for being able to watch movies on one or the other video source, you can do this with any source on a multiple monitor system. You just can't go fullscreen with a window in a secondary display (might be alleviated with twinview). The 'function's that can't be activated on a secondary display in a system usually are Directx acceleration , opengl acceleration, and ironically mulitmedia functions such as tv-in,tuning,and tv-out (hardware DVD may be in there too, but I haven't heard it yet). Since you're saying that a twinview obeys the rules of multiple display systems in windows 98, then a tv-out wouldn't even be able to be used as a secondary display , at least according to windows rules. Twinview is a new technology and doesn't have to follow the old rules exactly.
This is beside the point anyway as Geforce GTS's don't have twinview anyway. This is an MX feature only, unless someone has come up with a new hack. If anyone has a GTS and can display 2 different desktops on their monitor and their tv-out, please prove me wrong here .... cause I'd love to be able to do video editing and send a separate video window to my tv with a GTS and still have the program controls separately on my main monitor.
EDIT: I just tried using a DVD player on the secondary display from a Toshiba laptop with a Savage chipset. The DVD would not play on the secondary display, so probably add hardware DVD to the list of no-nos for secondaries. Movie files will play ok though. Twinview on Geforce MX cards may produce the same results of the Savage here.