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
sudo ifconfig en1 192.168.1.100 netmask 255.255.255.0 alias
(you'll need to change "en1" to match your adapter and "192.168.1.100" 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 !