Have you ever placed an MP3 file in your Sites folder on your iDisk?
Well if you do so, and if you copy the link into a web browser (I use FileChute to easily copy and paste the links to my iDisk files), the song will play in your browser through QuickTime.
But check this: if in your browser you change the first part of the link from http:// to daap://, the song will automatically stream in iTunes. Unfortunately it works with MP3 files only -- no m4a streaming, yet.
[robg adds: We've covered the daap protocol before in a few different hints, but none of them discuss using your .Mac account as a streaming server.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040421173439590