There is one minor exception; but it is highly unlikely to be what you want. Some Apple systems(I've seen it done with recent powerbooks, don't know about the others) can be booted in such a way that they will act as firewire hard drives. You could then daisey chain the powerbook, to the drive enclosure, to the main box. Doing this, though, would just give you access to the powerbook's main disk on the primary system, it wouldn't share the drive enclosure.
If you just want to share the drive, I'd recommend the following: connect the two computers with a firewire cable and set up a TCP/IP over firewire connection. Then just share the drive over that network. Overhead will be a touch higher; but it'll work.