Looks like switching to zfs was worth doing:

pool: jamespool
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://zfsonlinux.org/msg/ZFS-8000-9P
 scan: scrub repaired 0 in 29h13m with 0 errors on Fri Aug 17 13:12:16 2012
config:

        NAME                                                 STATE     READ WRITE CKSUM
        jamespool                                            ONLINE       0     0     0
          mirror-0                                           ONLINE       0     0     0
            scsi-SATA_WDC_WD7500AACS-_WD-WCAU47125177        ONLINE       0     0     0
            scsi-SATA_WDC_WD7500AACS-_WD-WCAU47162708        ONLINE       0     0     0
          mirror-1                                           ONLINE       0     0     0
            scsi-SATA_WDC_WD7500AAKS-_WD-WCAPT0622481        ONLINE       0     0     0
            scsi-SATA_ST3000DM001-9YN_S1F01R75-part1         ONLINE       0     0     0
          mirror-2                                           ONLINE       0     0     0
            scsi-SATA_WDC_WD7500AAKS-_WD-WCAPT0622515        ONLINE       0     0     0
            scsi-SATA_ST3000DM001-9YN_S1F01R75-part2         ONLINE       0     0     0
          mirror-3                                           ONLINE       0     0     0
            scsi-SATA_WDC_WD15EARS-00_WD-WMAVU2129350-part1  ONLINE       0     0 3.50K
            scsi-SATA_ST3000DM001-9YN_S1F01R75-part3         ONLINE       0     0     0
          mirror-4                                           ONLINE       0     0     0
            scsi-SATA_WDC_WD15EARS-00_WD-WMAVU2129350-part2  ONLINE       0     0   429
            scsi-SATA_ST3000DM001-9YN_S1F01R75-part4         ONLINE       0     0     0

SMART isn't seeing a problem with scsi-SATA_WDC_WD15EARS-00_WD-WMAVU2129350 yet though.