As MacGeneration notes, with a note on its website Apple announced Xserve will be discontinued and will only be available until January 31st. The note links to a PDF available here called “Xserve Transition Guide”.
Apple will not be developing a future version of Xserve. Xserve will be available for order through January 31, 2011. Apple will honor and support all Xserve system warranties and extended support programs. Apple intends to offer the current ship- ping 160GB, 1TB, and 2TB Apple Drive Modules for Xserve through the end of 2011 or while supplies last. Apple will continue to support Xserve customers with service parts for warranty and out-of-warranty service.
The Xserve was first introduced in 2002. Entire racks of Xserve systems are still used by many companies and universities in their networks. As Xserve gets discontinued, the Mac Pro Server goes live in the Apple Store. It can get configured up to 12 cores and ships in 2-4 weeks.