> the multicast server would continually send out the image, forever
> looping
> and , at the beginning it sends out a 'start of image' message. the
> multicast
> client would simply be told which image to listen for (from that IP, or
> via SAP)
> and , once it received the 'start of image' message, would save the
> incoming 
> image to file.  This way 50000 folk could download the 600mb file at
> the
> same time and the server would only ever see.... lets say 1Mb or 2Mb or
> traffic
> on its uplink. 

This is frequently done when ghosting a ton of computers over a LAN. 
Unfortunately it's not practical on the internet.