can you run sql server 2005 and sql server 2008 r2 in mirror mode?
The partners must be the same edition and the same version.

Please see BOL's reference on Database Mirroring Overview


  For a new mirroring session to be established, the partners and the witness, if any, must be running on the same version of SQL Server. 





  Since SQL Server 2005 Service Pack 1 (SP1), database mirroring partners and witnesses have been supported by Standard and Enterprise Edition. But the partners must use the same edition, and asynchronous database mirroring (high-performance mode) is supported only by Enterprise Edition. Witnesses are also supported by Workgroup Edition and Express Edition.