The unrestored log is the portion of the log at the mirror server that has not yet been replayed against the mirror database. Everything's been send across (it is synchronised mirroring), the replay is just very slightly behind. At 600k, I wouldn't worry, that shouldn't take more than a few seconds to replay if/when a failover occurs.
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