Phantasm66 has it right. Hard drives will always die.
MTBF has no meaning for an individual hard drive. It is a statistical projection based on all drives of the type... But does not know anything about the computer, the cooling, the frequency of use, and how full the drive becomes. It does not even have relevance for Government security computers. There are plenty of research articles about it. There is absolutely no way to predict the life of your own hard drive based on the MTBF estimate offered by the company which makes them. None. Otherwise, the price and the warranty would reflect it.
Hard drives are now less expensive per MB or GB than they have ever been, with the 40 GB drive faster at 7200 rpm, more reliable with S.M.A.R.T. technology, and under $40 in most markets... though not in your neighborhood store. SCSI drives commonly last for 7 years, but you pay dearly for that SCSI reliability and few computers other than IBM Intellistation and other IBM models are even designed to use SCSI... only networked systems do. You pay about 7 times as much per GB, and the SCSI drives are usually much smaller in capacity.
What counts most is the limited warranty provided. But even the warranty instructions say you have to backup your data.
Seagate offers a 5 year warranty. Samsung and Western Digital now also provide a 5 year warranty on some models.
We have seen brand new drives of every manufacturer fail within the first 30 days of use... and have them in our shop as collectors items.
A quality backup system is your only hope.