The firewire port on DV camcorders is an Output. However, there are devices you could use, such as the Quickstream DV ( http://www.mcetech.com/quickstreamdv/ ) which are better suited to what you want to do. Plus, as a bonus, it has a memory buffer that means when you hit record it starts the capture from several seconds *before* you pressed the button, and lets it run for several seconds afterward, meaning you are less likely to miss any of the action.
The firewireport on DV camcorders also accepts commands ( which are coming into the camcorder ). Firewire was designed to do peer to peer data transfers. In theory any firewire device can transfer data to another. It is not uncommon to download a video on a computer into a DV camcorder and store the video on miniDV tape. I was trying to find an "el cheapo" scheme for taking an ISight mounted in a dangerous place ( top of bike helmet for example ) and save the video somewhere.
The device at ( http://www.mcetech.com/quickstreamdv/ ) is very cool. Expensive too. It would be cheaper to buy a miniMac and do the same thing. I am thinking that maybe the way to go is to use a cheap Linux motherboard with firewire on it. At least the quickstream device gives me an alternative.
Yes, I suppose you *could* tell the video camera to record from its firewire port, but you would need to actually custom-build a device to somehow issue these commands over firewire. That means you'd have to have another gadget between the iSight and the camcorder that has at least some sort of computing power, and has been specially built for the task. A nice project for an electronic engineering student to try on the weekend, but not for the likes of us mere mortals, I'm afraid.