MacBook Pro - connecting an external iSight

Because at home I use a separate display with my new MacBook Pro unibody, the lid is closed and I cannot use the inbuilt iSight camera when connected to iChat or Skype. With the previous laptop I just plugged in an external iSight to the Firewire 400 port, positioned it alongside the display, and it all worked fine.

The newer laptop only has Firewire 800. I therefore purchased an 800>400 cable. When first plugged in, the iSight light comes on, and the focus can be heard working, then it shuts down again. Both iChat and Skype do not recognise that an external camera is attached. I have tried going through preferences etc.

Any thoughts on how to overcome this? (in the office I have a 24" apple display, and the in-built camera works fine).