I don't see why it wouldn't work, but it would have to be done a certain way. Many of the old PCI card slots ran on the USB bus anyway. They just used a different connector and some extra hardware. You're just going to need hardware in between the Firewire and USB to do the conversion. Some universal laptop docks now include firewire but interface with the laptop via USB.
I'm betting *somebody* out there makes a little device that looks just like a USB hub, but has 1394 on the input and USB on the output. It wouldn't surprise me at all if this cable Robert ordered has the necessary conversion hardware built in, just like a USB to RS-232 or USB to Parallel converter. Those aren't just magical pin-to-pin adapters. There are chipsets in line that make the conversion possible.
Saying that its simply impossible is just silly. Its just a matter of the right hardware, and the right software. Plenty of USB devices stream, there's nothing magic about firewire that makes it the only option in that realm, so there's no reason that such a conversion wouldn't work just fine for a video camera.
EDIT: After a bit of research, I can say that such devices do exist, but they're hard to find and rather expensive. Here's a cheaper alternative.
This device plus
This device.
This should work for laptops like my Toshiba that have no express card slot, or the slot is already in use for other stuff.