Well I don't own a webcam... but I was able too see other people with MSN when they send me a webcam invitation... but since a few a days ago I can`t... What can it be, I have no idea?
If you are using a fw you aren't letting messenger connect to the correct ports, there are about 12 or so you need to open.. The fault could be with the inviter too though...
UPDATE : I have made some tests with diffents laptops using the same connection here at home and I have the same problem... The problems seems then my Internet connection, but what could it be?