I just saw this mentioned on TUAW: Musicast is a Mac OS X application that lets users stream their iTunes playlists on the web. You simply install the $18 application, choose the playlists you want to make available, and Jane User on the internet can listen to your playlists as streams.
I downloaded the demo app and installed it. I got this message:

Fair enough. Here's what it looks like on the desktop:

Musicast recognized my iTunes playlists right away. All that was left for me to do was check the playlists I want to share and point my friends to my HTTP stream. The firewall here in corporate-ville blocked the outbound stream, and trying to connect to my Musicast resulted in a timeout.
So, are you thinking what I'm thinking? Unlicensed sharing? I know that when I installed the application, I agreed not to share any copyrighted songs. But, other than clicking "I Agree", and the associated guilt should I disobey, what's stopping me? I turned to the Musicast FAQ and found this nugget:
Ah, I see. Even I choose to break the law, the RIAA probably won't catch me. That gives me warm fuzzy feelings.