Does the Arduino need to be involved in the USB video feed? It sounds as if a plain old USB web cam would do what you need. I don't know what your 'parallax propeller' is but if it's capable of receiving a serial video stream over USB and also capable of talking to the Arduino over USB then I'd have thought you have the basis of a solution. With this approach you would connect the web cam and Arduino separately and design your application so that it reads the video stream and sends commands to the Arduino as required.