(There isn't too much information really out there, so I'll tell you as much as I know about it from experience)
It's basically a hardware based SHOUTcast server. It's what the AOL Hosted Streams run as well as Radio@AOL and the newer Radio@Netscape. Besides the URL (http://ip:80/stream/####
) you connect to it like any other SHOUTcast stream and usages should be transparent.
Though I think there is some fancier things you can do when connecting to it with some headers. At least the header response is a little different then a normal SHOUTcast server when you connect to it. And it can be treaded differently if you connect to is as Ultravox, but I am not sure if any of those extra headers make a difference in anything.
I believe newer versions (which is use on the winamp.com videos and music section) is now called AOL Streaming Server (ASS).
From a developer's or listeners standpoint any player the can play SHOUTcast streams can play streams from an Ultravox server.