on the face of it they seem to be competing interfaces, but then again, it's hard to say if that's the case.
USB doesn't seem like it's going away any time soon - after all, it's rather new, and a replacement for legacy parallel/serial ports...a rather decent one too...they're also, according to the register or inquirer (can't ever remember between the two) moving away from host interface to accomodate the peer-to-peer trend..that is, so two USB devices can interact w/ eachother without the need for a cpu. - this would be a benefit for sure.
u can now get virtually any peripheral in USB...firewire will probably be reserved primarily for the digital video market...of course it's also excellent for high-bandwidth areas, like external storage, where it's penetrating quite well...i have a feeling that USB 2.0 will only be left for low/medium bandwidth peripherals, while the next iteration of firewire will dominate the high bandwidth market.