Well, today I quickly wrote one which you can find attached in the related bug report #320233.
It turned out to reveal a couple of issues with the involved components so consider it very experimental and ugly.
For those who still want to try it extract the archive from the bug report it to your ~/.gnome2/rhythmbox/plugins/ directory, restart Rhythmbox and enable the “Stream Recorder” plugin.
Make sure your GStreamer installation has the plugins “lame“, “id3v2mux” and “gnomevfssink” (check output of “gst-inspect“).
It adds a button in the toolbar which if enabled, will cause a file being written onto your desktop for anything Rhythmbox plays back.
If you change the song entry it will create a different file. For radio streams, splitting individual files does not work yet (gnomevfssink does not allow setting location property when the pipeline is in PLAYING or PAUSED state) and you have to toggle the recording button if the metadata for the radio station changes to cause the plugin to write to a new file.
As noted it still has issues so toggling recording while playing back might screw up things. It also only encodes the stream into a MP3 file. Using the selected GNOME Audio Profile of the user instead is something which needs to be implemented yet.
This plugin should provide a testing ground and allow to fix all issues with using the tee recording functionality in GStreamer and Rhythmbox.
Update 17/12/07: The latest version now uses your currently set GNOME Audio Profile for encoding (thus MP3/OGG/AAC…), writes ID3 meta-tags and automatically splits tracks from radio streams.