With the help of Kuma:

http://groups.google.com/group/microsoft.public.windows.powershell
/browse_thread/thread/9b10ea1270dfd0ad/eb78a0bc226837d6#eb78a0bc226837d6

I have picked up this gem, which gives all the physical IPs on a system:

0..1 | %{([System.Net.DNS]::GetHostEntry(""))
.AddressList[$_].IPAddressToString}