Database Mirroring in SQL Server 2005 works this way.

The Primary, witness and Secondary forms a quorom of three after a successful setup. A fail over will occur if Witness and Secondary agrees that Primary is dead, so they will form a quorom of two and automatic fail over will initiate within 3 seconds. Your secondary/mirror will act as the Primary/Principal. Now, once you restart Server 1 which as the original principal server before the failover it will find out that the mirror server (server 2) is the acting Principal server, so Server 1 will take the role of Mirror server this time. In this case you need to manually failover to server 1 by executing this statement on Server 2

"ALTER DATABASE database_name SET PARTNER FAILOVER" .

This TSQL statement will revert the Principal role to Server 1...