I'm having the same problem with a backup server we have here (Supermicro 5015M-MT chassis, 3ware 9500S-4LP RAID card, 4 WD5000KS drives in RAID5, no spare). Array performance is degraded, and drives drop from the array randomly (about every 2 weeks at this point). 62360 KiB/s block output, 88127 KiB/s input on the WD system, while I have an identical system, but with 500GB Seagate drives instead, getting 78017 KiB/s output, 122658 KiB/s input.
I can recover by pulling the "failed" drive's tray out of the chassis, re-inserting it, and telling the 3ware controller to rebuild. Still, it's a pain. We run almost 100% Seagate around here and have been really happy, so these WD drives are starting to get on my nerves. Hopefully WD will get updated firmware out (but from what I've seen in their replies above, it doesn't sound likely).
Thanks for the info all.