I found this thread, about how to use AUNetSend waaaaay back in 2005- http://www.macjams.com/forum/viewtopic.php?forum=6;showtopic=28700
Double click on any instrument track in GarageBand to open up "Track Info" screen. Choose an additional effect from the area below "Compressor" (it should say "None") Go all the way down to your Audio Unit Effects and you should see "AUNetSend" and "AUNetReceive". Depending on who is doing wha,t if you are the sender, the other person has to pick the "AUNetReceive" option. Now since I don't have Tiger, this is were I get lost and don't know how this actually works from here, but if 2 people would like to experiment and fill in the blanks I'm sure a lot of people would really appreciate it!
How do I do this today? Well... should I say how do I do this a month ago, since I haven't upgrade to iLife '11 yet.