As I understand it, Desktop Macs and iMacs do not supply power through their Firewire ports when the computers are powered down, even though they may themselves be connected to AC power.  MacBooks and MacBooks Pro, however, continue to supply power through the Firewire ports, as long as their AC adaptor is connected to the machine and to AC power.

There was a thread here on the RME forum some time ago, which included some pictures of a fried Firewire port - the result of a spark wreaking havoc, when hot plugging/unplugging a Firewire device.  Apparently the Firewire controller chip is very small, and located very close to the connector(s), and so is easily damaged by a static zap happening at the connector.

For safety's sake, I always wait until my iMac is powered down, my Fireface too, before unplugging the Firewire connections.  On my MacBook Pro, I will disconnect the AC adaptor as well.  When I think about it, my whole livelihood flows through that little connection, so I'll baby it.  During my last visit to the Genius Bar at an Applestore, I saw several of the technicians hot plugging bus powered firewire hard drives (LaCie Rugged) to people's computers, in order to reboot into one of their diagnostic environments.  I insisted that they wait until I had completely powered down my machine, before making any connection, and of course they were fine with this slight delay.

A word to the wise is a penny earned, as they say...