I have a NAS that I regularly upgrade with new hard drives when the old ones' warranties expire. I just recently went from 4x 1 TB + 4x 750 GB in two RAID5 arrays to 8x 2TB in RAID6. That NAS is backed up off-site to another, similar NAS at trusted, non-corporate location, but only the parts of it which are 100% irreplaceable: pictures, video, financial paperwork, schoolwork, etc. The drives in the second NAS are from different batch and are 4x 3TB in RAID5.
I've considered also paying for a service like Amazon Glacier to archive those really important things, but the price still seems too high for the amount I have to store and I have concerns about the security of it. Tarsnap is a crowd favorite, but you certainly pay for its paranoid level of security. I'm eager to see what comes of Maidsafe and Storj, which are distributed systems to which I could certainly lend a whole lot of spare space.