I got so excited today. Almost managed to get VirtualPC working with an Airport connection (rather than Ethernet) today, but… not quite :(
Hopefully this will help … maybe someone get get it working…

First up, add a second IP address to the Airport adapter (multihoming is only supported in OSX 10.3 and beyond apparently)… in Terminal type
<pre>sudo ifconfig en1 netmask alias</pre>
(you’ll need to change “en1” to match your adapter and “” to match the IP address you want to use)
Then set VirtualPC to use Virtual Switch / Aiport and re-start the OS (if needed)
Set the ethernet adapter in the Guest OS to use the IP address / netmask as above and relevant gateway and DNS servers for your setup
.. I was able to ping the outside world from within the Guest OS, but even with the OSX Firewall turned off (not a good idea) still no luck in getting a web connection :(

I’ve submitted support requests with both Apple and Microsoft, but somehow I’m not expecting a solution any time soon !