I just got this working today, so if you are still struggling here is how to get it to work.
First, set it all up, this only needs to be done once.
1. Install Nicecast
2. Install the Soundflower from within Nicecast (you only have to do this once), restart, and in the System Preferences choose Sound, and under Input select "Soundflower (2ch)"
3. Open the Server window from within Nicecast (Window>Show Server) stop it, and change the Address to your local IP address. start it again. (not sure if this whole step is needed at all)
3. Choose "Spotify" as the source (you'll need to have it playing to do this)
4. within the finder, select the PS3MediaServer application, and control click it and choose "Show Package Contents". Within the new window go to Contents>Resources>Java and open the file "WEB.conf" in text edit
5. add the line
(without quotes) under the "#shoutcasts" section. Save and close. (Notice the URL - it should be your local IP address, and NOT have the "/listen.m3u" at the end)
6. Quit everything, its now set up.
Now the order in which everything starts up seems to play a big part in making it work.
1. Open Spotify, start playing music.
2. Open Nicecast, start the broadcast (Spotify should still be the source)
3. Open PS3MediaServer, let it find the PS3
4. On the PS3, select Music>[name of your PS3 MediaServer share]>Radio>Spotify.
Thats it, it should work!
If you don't want to listen to the music on your computer because of the sync issues, just choose Soundflower in the System Prefs>Sound>Output section.
Hope this is of some help...