In 2008 a mirroring setup can only have one principal and one mirror. End of story.
SQL 2012 (Denali) has AlwaysOn which allows for multiple active replicas. This is a new feature, it's not an enhancement to mirroring, SQL 2012 will still have the old mirroring feature as well
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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