One of the nice things about getting married is that I get to combine all of the geek toys that I have, with all of the geek toys that I gave my girlfriend/fiancé. (The look on her face when I gave her the Airport Express was tragically similar to Marge's when Homer gave her a bowling ball with his name on it for her birthday.) Our co-mingled household includes the following: PowerBook and iPod for me, WinTel notebook and iPod mini for her, three Airport Expresses, and my old Sawtooth G4. We wanted to consolidate our music on the Sawtooth to make backup and music management easier. In order that we could both have full control over our playlists, I placed the music on a shared drive on the Sawtooth, and then managed our iPods through separate accounts on that machine (the only side effect was that her iPod had to be reformatted). So far so good.
Then I had the bright idea that it would be cool if we could also be able to simultaneously access our playlists via Airtunes (each of the the Airport Expresses are connected to a different stereo -- Office, Living Room, Bed Room). On the Sawtooth, I logged into both accounts and clicked the "Share my music" checkbox under iTunes: Preferences: Sharing. I left both accounts active and running iTunes. On the laptops, I made sure that "Look for shared music" was checked under iTunes: Preferences: Sharing. Then I looked on my PowerBook and saw that under Shared Music in iTunes, there were entries for me and my girl. The problem was that I could only access one of the iTunes shares at a time.
Whenever I accessed one iTunes share, any attempt to access the other was met with an error message saying that share could not be accessed and that I should make sure that sharing was enabled and that the firewall was not blocking the traffic. I confirmed that both accounts on the Sawtooth had iTunes sharing activated and the firewall under System Preferences: Sharing: Firewall had a check in the "Allow" field next to "iTunes Music Sharing." I was stumped. Here's where reading comprehension, or just being observant, would have come in handy because iTunes was giving me a very helpful error message telling me that a specific port needed be opened.
After much trial and error, I finally noticed that the port that iTunes wanted opened was different than the default port under System Preferences: Sharing: Firewall. The solution: I went to System Preferences: Sharing: Firewall on the Sawtooth and then selected "New..." in the drop down, changed the Port Name to "Other," and then filled in the "TCP Port Number(s)" field with the port that iTunes was requesting and gave it a description, e.g., iTunes Music Sharing 2. The final result being that the Sawtooth now has two ports open for iTunes: the default, and additional port as given in the error message.
As I type this, I'm in the Living Room listening to one of my playlists and the spouse is in the bedroom listening to one of hers -- all of it being streamed from the same old Sawtooth G4 up in the loft.
FYI on simultaneous streams from the Sawtooth. I'm not sure how taxing this is on my music server, but I should point out that it has been upgraded lest someone try to use an old machine for this purpose and find it not up to the task. Specifically, the Sawtooth has one Gig of memory, an upgraded 800 Mhz G4 processor and an 802.11g PCI card that OS X recognizes as an Airport Extreme card (after much trial and error, but that's a story for another time).

