Actually I remember a few months ago (back when SHOUTcast had it's own forum) I mentioned a system that would work but Tom didn't like the idea.
Basically it would let the listener decide what type of connection to stream, then when the listener would call into the server, it would send one header peice saying what bitrate they wanted, that would inturn cause the DSP to send that content, to that listener, so those using 24k streams can listen to broadcasts that will go as high as 128kbps. Then the broadcaster has the control at what the highest bitrate to send out, i.e. I have a cable modem so the fastest I want to send is 56kbps, so I tell the DSP that, and when the person tunes in if they chose something over 56kbps I would only send them the 56k stream
didn't understand why the concept wouldn't work but I guess Tom has his reasons.
"Broadcasting with the Web in Mind"