It has been announced at WinHec by Bill Gates. Windows Server 2008 will be the official name of Longhorn Server. Will that mean that Windows Server 2008 will only be available in 2008? Probably, honestly I don't know, what I know is that RTM is still planned for H2 2007 and it might be that the general availability will be somewhere in 2008.
Normally when Microsoft is releasing a product after August 1 in a year it will be named 200x, where x is next year.
Longhorn is to be released after August 1 therefore it will get the name Windows Server 2008.