Actually you've already got it.  Since Server 2008 is the same codebase as Vista SP1, it technically already is SP1 as it ships RTM.  So the first service pack for Server 2008 will be SP2.

For full details and a great backgrounder on how this all came about, read over this blog post from Iain McDonald.

http://blogs.msdn.com/iainmcdonald/archive/2008/02/15/windows-server-2008-is-called-sp1-adventures-in-doing-things-right.aspx

When I think of Heroes happen {here}, Iain is one of the people who put the rest of us between those brackets.  A hero's hero.