Once and for all, the difference between the WD4000YR and the WD4000KD is simple marketing.
They are the exact same hard drives and come off the exact same assembly line. That's why Western Digital is so vague on the differences between the drives. They don't want to screw up the marketing of their "enterprise" line by saying it's the same basic thing as a "desktop" drive. I don't blame them... instead of people thinking "wow, this 'desktop' drive is just as good as their 'enterprise' drives!" the perception would be "lame, they're just taking a plain 'desktop' drive and labeling it as 'enterprise class'".
The ONLY physical difference between the two models is that the "enterprise" class WD4000YR limits the error checking to 7 seconds because they're supposed to be connected to a RAID card that will take care of the error checking. Your cheesy little RAID-1 chip built into your motherboard is not good enough. They're expecting you to connect it to a real RAID card, something that can do RAID-5 type setups in the hardware.
The extra couple of dollars you pay for the WD4000YR is basically paying for a 5-year warranty instead of the 3-year warranty that the WD4000KD has and some extra testing. The YR goes to a longer "burn in" or testing time than the KD does before it leave the factory. That allows them to weed out a higher percentage of problem drives and claim an higher mean time between failures.
So anyone buying the WD4000YR with no plans of hooking it up to a fancy RAID controller and using it in a RAID array is being silly. Just use the WD4000KD to try to avoid the possibilities of losing a huge chunk of data because of the lack of bulit-in error checking in the YR... IF it ever starts to develop bad sectors. The KD will be just as reliable (or not) as a YR drive if you plan on using it 24/7.
If you're really that worried about having a drive that is less likely to die, then your only real choice is to go SCSI. For most of us, it's just not that big the big bucks for SCSI disks. Just keep current backups and you'll be okay.