Windows Server 2008 and Windows Vista SP1 both hit RTM on Monday, and already there seems to be some confusion around about why Windows Server 2008 has Service Pack 1 mentioned in winver and System Properties.
This is normal. Something similar happened with the 64-bit version of Windows XP, it was SP1 out the box because it was built on Windows Server 2003 SP1.
In this case Windows Server 2008 isn't built on top of Windows Vista SP1, it is Windows Vista SP1. They both have the same code base, and as a result the same build number 6001.18000. The only difference is the applications and features built on top. That means the same patches will apply, and even service packs going forward.