If you just upgraded your database to SQL Server 2008, you want your database to be compatible with SQL 2008, of course. To see which compatibility your database currently has, just type the following query (new in SQL Server 2008):

SELECT * FROM sys.databases

Under column compatibility level, you will find the answer.

80 = SQL Server 2000

90 = SQL Server 2005

100 = SQL Server 2008

To change the compatibilty level, just this command:

ALTER DATABASE database_name
SET COMPATIBILITY_LEVEL = 100

For a complete reference go to:  http://msdn.microsoft.com/en-us/library/bb510680.aspx