Slashdot discusses options for long term data storage. This comment lists some of the previous related discussions that have happened on Slashdot. The consensus seems to favor multiple copies on hard drives with periodic testing and migration to new drives before failure takes place. This article gives a formula for calculation of the Mean Time To Data Loss (MTTDL) for multi-drive arrays based on the number of drives, their expected mean time to failure, the degree of redundancy (one or two independent parity channels) and the time to replace a failed drive and rebuild (which can be measured in days if you don't notice a failure right away). With this approach, using the Seagate 1.5TB drive with a quoted annual failure rate of 0.34% less than 2% of these drives should fail in 5 years (the warranty period) so taking 5 years as the mean time between failures should be very, very conservative. Then, if you have a RAID array with 3 data and 1 parity disk (or a 4 drive RAID 5 system) and it takes you a week to detect and replace a failed drive then the MTTDL would be (working in days):
(5*365)*(5*365)/(4*(4-1)*7) = 39650 days
or about 108 years before you had 2 drives die within the 1 week replacement window and lost your data. Alternatively you might use each drive as a simple redundant copy of some data, so if you have 3 drives you put the same data on each, then once a month you check each to see if it is still fine (perhaps you put more data on it at that time as well), then using the same conservative 5 year MTBF you would have:
(5*365)**3/(3*(3-1)*(3-2)*(31)**2) = 1,054,178 days
or 2888 years before you had all three drives die within the same 1 month window and lost your data. So it looks like just putting your important data on two or three external hard drives which you periodically test and refresh should be safe enough, and the more copies you have then safer you will be. Of course, with multiple copies you can place some of them in off site storage which will help protect against fire, theft, flood and other catastrophes.