Rich,
Whiteboard uses port 1503 TCP, which needs to be enabled to traverse any firewalls. Since
a specific port is used for the TCP data connection (1503), if the client is behind a
non-UPnP NAT device, the port must be mapped to that client. This ensures communication
through the least common denominator NAT device. This also means the port cannot usually be
used by another client behind the NAT device; only a single client behind the same NAT
device can have an AS or WB session active.
Source/More Info:
http://www.microsoft.com/technet/prodtechnol/winxppro/deploy/worki01.mspx