The firewire interface chips have input protection diodes that shunt over / under voltages to supply/ground. The problem that can arise when there's a good amount of current behind the overvoltage is the entire supply voltage gets raised and multiple devices get smoked as a result. Well designed firewire cards have current limiters on the 12V supply being fed to the 6pin sockets. I'm inclined to doubt the firewire ports on mobos have such extravagences considering the better firewire cards can cost as much as some of the cheap mobos.

Apple to their credit seem to be dropping 1394a in favour of 1394b (Firewire800). Not only is 1394b twice the speed the connectors are vastly superior. One of my PCs has a firewire card with 2x 1394b ports. Lovely stuff, the 1394b 9 pin to 4 pin cables are so much better although a bit harder to purchase.

I'll say again, the only firewire cables to buy are the ones made by Sony. The cheaper ones are generally too rigid, this is a problem if you're using your camera as a VCR, you're probably putting too much strain on the 4 pin socket in the camera. Mostly all that holds them is where they're soldered to the PCB. Enough flexing and a soldered joint fails leading to wierd problems.
Worst of all the cheap 6pin to 4pin cables have the conductors from all 6 pins active right through to the headshell of the 4pin plug. The surplus 12V power conductor is simple cutoff leaving an exposed end. When the join between the moulded headshell and the cable fails that 12V conductor can end up shorted to a signal conductor or pin and then things can start to smoke.

Bob.