AppleInsider reports on multiple files in the recent builds of Mac OS X 10.5.7 that point towards the imminent update of Apple's Xserve range of servers.
A reader of AppleInsider pointed out that extension file AppleTyMCEDriver contains a reference to "Xserver3,1" a machine ID which is one major upgrade up from the current "Xserver2,1" models.
If this wasn't enough confirmation of a nearing Xserve update, AppleInsider points towards references to Marvell's Yukon 2 ethernet controller. Currently Xserve models use the Yukon 1 controller.