The only failproof backup, as was stated, is to have your critical stuff in multiple locations.
Take me for example. The things I can't live without are stored on my desktop PC, which is backed up to WHS, and also has a mirrored placement on my Server 2008 R2 box, which has RAID 5. On top of that, I burn all critical things to either BluRay or DVD DL. It's not impossible, just highly unlikely, that all of my backups will go down at the same time, so I'm relatively safe.