One of the features prominently touted by Apple in the new XSan 2 is its support for 3rd party storage solutions. It looks like there is a very good reason for that: Apple is no longer making the Xserve RAID. That's right, if you head on over to the XServe Raid's former home on Apple.com you're greeted with some information about the Promise VTrak E-Class RAID Subsystem, and a link to find out more on Promise's website.
It would seem that Apple wasn't making enough money in the fast paced Enterprise storage market and is bowing out. This makes lots of sense to me, since (for the most part) storage is a commodity market, and that is something Apple has never been interested in. Now the company can focus on making the software that one uses to power and manage your SAN (i.e. XSan 2) the best in the market (and judging from the screenshots it looks a lot better than most SAN software I've used).