Internet video and audio streams
If you want to listen to radio streams you can do it :) (Videostreams should work as well.)
There are two ways this can be done
Create a file with the extension .strm and fill it with the URL of the stream. This should look like: mms://host/path/to/stream or rtsp://host/path/to/stream, though other protocols are supported such as http://host/path/to/somefile.mp3. Put the .strm file somewhere and open it with the XBMC.
Download a supported playlist file off the internet, for example stream.pls or something else (these playlist files can often be found on the website of the radio station). Save it to a network share or supported device (or your xbox). Play (remember if it doesn't work and it's a streaming media file then....it's NOT SUPPORTED!)...