But, your ethernet cables plug into something - DSL or Cable Modem. Many of them have firewalls, which puts your ethernet connection on a LAN.
i.e. Router.
note: Some ISP's may block some streaming content ports. I know of one, in particular - Clearwire wireless internet. But, in either case the problem is most likely blocked ports.
my experience with video chatting between AIM and iChat has been very difficult. very rarely does it work and occasionally i can get audio, most of the time however it doesnt work. i just video with other people using ichat, good luck with it