Discovery through adversity once again. After applying the AirPort 3.2 update on my Powerbook G4 1GHz (sans-AirPort extreme), I discovered Virtual PC 6.1 with virtual switch (DHCP) could no longer "see" Mac OS X and vice-versa, both on the AirPort interface logged into an AirPort network. This lead to desperate experimentation that panned out.
Caveat: no testing beyond my experience of AirPort 3.2 update / Panther / VPC 6.1. AirPort card and built-in Ethernet required. Mileage probably will vary.
Situation: I deploy struts apps into Tomcat under OS X that utilizes JDBC to a SQL Server, as well as a socket-based Java interface to a Business Intelligence server, both running under VPC 6.1.
Here's how to get this to work with no Ethernet cable attached to the built-in Ethernet interface and having not joined any AirPort network (although AirPort interface must be on). It doesn't get more isolated than that in a car traveling down the interstate!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031101210620201