I think the Xserve has one slot for a PCI card, which could be the fiber interface card.
Fiber channel is usually for really big shops, with multi-terabyte storage arrays from EMC, Hitachi, etc. I don't think shops like that would be interested in a dinky little Apple RAID box.
So, FW800 would be a better interface for building a smaller Xserve set up for companies with somewhat smaller storage requirements.
It's astonishing how long this product is taking to come to market, though, isn't it?