You are probably correct about the original controller not being supported by XNA. I took another look, and it isn't DirectInput that is required of the device, it is XInput (the replacement for DirectInput). So this means that XNA doesn't require the 360 controller per say, but it does require a controller that was built with xinput in mind, which I would imagine the companies that still make these devices for games (like logitech) support now in their new devices.

So it does look like to use the old XBOX controller, you would have to find or create some sort of DirectInput wrapper to expose it to .NET.

Or you know... you could get a 360 controller