Yes, but it's not supported except during an upgrade scenario. (upgrading a 2008 mirror pair to 2008 R2). If you ever fail to the mirror, you will not be able to fail back without upgrading the principal to R2.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability