Try:
select * from Win32_OperatingSystem WHERE Caption LIKE "%Server 2008%"
ProductType 1 is for Client OS, If you're targeting Domain Controllers (2) and Member Servers (3):
select * from Win32_OperatingSystem WHERE Version LIKE "6.1%" AND ( ProductType = "2" or ProductType = "3" )
The above can be shortened to:
select * from Win32_OperatingSystem WHERE Version LIKE "6.1%" and ProductType <> "1"