I would guess that the problem come about because of the protocal that Messenger use's to do voice and video. Its the H323 protocal.
The problem is with ICS all IP address inside your network (and rightly so) look like they come fron the single machine that is shareing the connection. This coupled with the fact that part of the H323 protocal requires the use of dynamic ports means that your going to have a night trying to get it to work.
If there's only one machine your ever going to be using the webcam on then you 'could' foward the range of ports back to that computer from the ICS one but I'm not sure if or how you can do that with the built in ICS software.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS