ActiveWin are claiming that the final designation for Windows Codenamed Longhorn Server will be Windows Server 2007 after all, how original! Speculated here long before Vista hit RTM.
"An unnamed Microsoft official just confirmed to ActiveWin.com that the new Windows server succeeding Windows Server 2003 will be named Windows Server 2007, now known in its beta form as Windows Server "Longhorn". Windows Server 2007 is currently slated to be released in the second half of 2007, culminating a long beta process which began in early 2005. In addition, development has begun on the next series of MSCE certification tests geared for Windows Server 2007."
An interesting point to note, Longhorn Server is not expected to be completed before the start of the second half of 2007. We all know how good Microsoft are for keeping dates, so anything could 'still' happen. For now at least it's Windows Server 2007.
News source: ActiveWin