Apple is abandoning enterprise? I guess I missed the press release. I suppose you mean the Xserve, but the death if the Xserve does not augur anything really, especially with Thunderbolt’s advent. The dev preview of OS X Lion includes Xsan, so there is no reason to believe Apple is abandoning it. Indeed, Apple specifically mentioned Fibre Channel in their feature page for Thunderbolt, and that can’t be an accident. Are your Xserve MDC machines very old? You should look into ActiveSAN as an option when it comes out.
Now, given that you already have the Xsan Fibre Channel infrastructure and the clients in place, all you would need to buy to scale up is more RAID arrays. I can’t see how you could find a less expensive storage option to meet your needs than expanding your existing system when you already own the stuff that makes Xsan more expensive than the Ethernet options and it is still good.
Best,
Andy