I have made a tutorial for making a quick screen cast from one Mac to another - without the need for any extra hardware apart from a Firewire cable.
This way the owners of two Macs will be able to create a digital video mixer on the fly using the screen capture of one computer as a video source in e.g. Modul8 on the other computer.
Yea I know it's not a real substitution for a video mixer but it might be a worthy alternative to a TV capture card.
http://www.udart.dk/2009/07/30/the-5-vi ... -firewire/
Let me know what you think