Additionally, the only time that mirroring is supported between different versions is as part of a rolling upgrade. To be honest it makes no sense any other time, because you can't fail back (because you can't restore, even log records, down-version) and hence a HA/DR solution where you can't fail over is a bit of a pointless solution.
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