I have a FreeBSD 8.x machine running ZFS and with a 3ware 9690SA controller.

The 3ware controller shows an ECC-ERROR with one of the disks:

//host> /c0 show
VPort Status         Unit Size      Type  Phy Encl-Slot    Model
------------------------------------------------------------------------------
p0    OK             u0   279.39 GB SAS   0   -            SEAGATE ST3300657SS 
p1    OK             u0   279.39 GB SAS   1   -            SEAGATE ST3300657SS 
p2    OK             u1   931.51 GB SAS   2   -            SEAGATE ST31000640SS
p3    ECC-ERROR      u2   931.51 GB SAS   3   -            SEAGATE ST31000640SS
p4    OK             u3   931.51 GB SAS   4   -            SEAGATE ST31000640SS

/c0 show events shows no ECC errors in it's recent history.

ZFS does not currently detect any errors. zpool status says No known data errors

My question: Is this ECC-ERROR something that I need to be concerned about?

According to the 3ware CLI 9.5.2 Manual, an ECC-ERROR means that the 3ware controller caught a read-error for one or more sectors on this drive. This sometimes occurs when a RAID array is recovering from a failed disk. I believe that ECC-ERRORS can also be detected when the 3ware Controller verifies each disk. None of the drives have failed and thus there was no drive rebuild, so I assume that 3ware discovered a bad sector when it ran it's weekly auto-verify scan of the disks. Is this a safe assumption?

According to our logs, ZFS has not detected any bad sectors on this drive. ZFS can work around read errors -- if ZFS detects a bad sector on the drive, it will simply mark that sector as bad and never use it again. From the ZFS perspective one bad sector isn't a big deal, although it might indicate that the drive is starting to go bad.