I have not understood why people were not able to use iChat's Rendezvous mode to communicate with me. People stated they sent me a message, but I would never receive it. Turns out it was, in fact, my Apple built-in firewall blocking incoming iChat messages. This does not seem like it would be a hidden hint, but you would have thought Apple would have set iChat as one of the "common ports."
To allow incoming iChat connections, go to the Sharing System Preference panel, click on the Firewall tab, then click the "New" Button. Click on the Port drop down menu and select "Other." Enter 5298 for the port number. Optional, but recomended, enter "iChat" for the description.
[robg adds: I have changed the port number (it was 5289) and any references to Rendezvous, as this is really an iChat hint (see the comments below).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030408172328173