I have just come across a Lego Mindstorms FAQ which details a great more information about the NXT and what it will be able to do, including burning questions about using it as a USB host - which Lego say not.
The device is purely a USB device, and cannot be configured as host. That is not to say that the clever community don’t come up with some hack to get more out of its USB connector, who knows. One suggestion is a simple host board, which requests a particular file from one of its devices, and then uses that as a set of instructions to act upon. This would mean a