Ah, Firewire. And we thought SCSI was full of voodoo.
First try some other FW devices to make sure it's not just one bad one.
I work on a lot of sick drives, and have had a lot of Firewire ports, both in devices and computers, get fried. Now I only use FW when forced to (like to boot a PPC to an external drive), and use a FW isolator cable--one that does not transmit power, only data--to protect my diagnostic drives.
I have been able to resuscitate a couple of FW ports on computers (a Cube, for example) this way: perform a "programmer's reset," the same way you do when you are applying a firmware update. I think on some MacBooks and MacBook Pros you start with the computer off, then hold down the power button till it flashes quickly. From there the computer will reboot on its own.
You should also try resetting the PMU or SMU. And definitely try booting to an external to make sure it's not a software problem with your installed system.