Hmm... an interesting idea:
So in a few years when Firewire is at 1600 and we've got 4 ghz G5's (or G6's), what if Apple made an entirely new type of computer, one that's totally modular. You'd have a small base containing the processor, ram, display card, and cooling. You would then have a monitor port and multiple firewire ports for hard drives, ipods, whatever. Think of it...with only processor, ram, and display card, they would be cheap. Schools could buy them and put them in classrooms. Each student could carry a HD with them, and just plug it in and boot up.
Ok, just a little glimpse of the future. On a more concrete note, and all-firewire system would (or at least could) be cheaper to build, assuming you could get production of internal firewire hard drives into volume production.