Windows Server 2008 is about to be released, with a twist: it already carries the first service pack!
Why are people making such a fuss about it? First, let take a look at SP definition according to Wikipedia:
A service pack (in short SP) is a collection of updates, fixes and/or enhancements to a software program delivered in the form of a single installable package
So the question is: can Windows Server get updates, fixes or enhancements prior to it's launch? Well it can, through Vista, sharing the codebase over the kernel.
How is this visible? Launch winver.exe both over Vista SP1 and Windows 2008 Server SP1, and you'll get the same version: "Version 6.0 (Build 6001: Service Pack 1)".
We expect new OS versions to introduce new problems and SP to solve them. So can we expect Windows Server 2008 to be more reliable because it comes installed with a paradoxical service pack? I believe so, sharing the kernel codebase is a guarantee of stability. The Vista SP1 vows for Windows 2008 Server. Linux does the some over their kernel, different distributions share a kernel on which people trust.
So what do we expect from Windows 2008 SP2? Probably it will be coincident with Vista SP2, and this will go on until a new server generation arises, or until Microsoft branches their OS versions from client and server.
Here's a cool post about it.