I've previously shown how to setup Database Mirroring in SQL Server 2005 with T-SQL, but how do you failover to the mirrored databases using T-SQL?  Here's how:

--Run on principal
USE master
GO
ALTER DATABASE dbName SET SAFETY FULL
GO
ALTER DATABASE dbName SET PARTNER FAILOVER
GO

--Run on new principal
USE master
GO

ALTER DATABASE dbName SET SAFETY OFF
GO

If you are using synchronous database mirroring, just ignore the "SET SAFETY" commands. 

We use asynchronous database mirroring for performance reasons as our sites are 300 miles apart.  Even though we have a fast network in between the two sites, the latency is too high when we tried it in synchronous mode.