In SQL Server 2008 -- after you have restored your SQL Server databases to the mirror server and setup the connection between the principal and mirror instances, then you will discover that the database will stay in a "Restoring..." state.

 In SQL Server 2005 it said DatabaseName (Mirror, Synchronized / Restoring...), but this behaviour has changed for the 2008 version, where it show says: DatabaseName (Restoring...).

This is not a configuration error and the mirroring is configured correctly (as you can se if you right click on the database, select Tasks and Mirror...).

Have a look here for more info:

http://support.microsoft.com/kb/956005