I have just ordered two 500GB drives, and intend to put them in a RAID1 array. Not technically a backup, but in my eyes the biggest threat to my stuff is drive failure.
Why do people say RAID1 is not a backup system? Because it doesn't protect against acidental deletion, natural disasters and theft. If I'm being honest, natural disasters and theft will most likely result in my external hard drive going too. And in a robbery a robber isn't going to run out of the house with a big freakin' full size case PC, he's going to go for small and valuable items (like my DSLR

). Accidental deletion? Well, I keep original unedited copies of all my files separate from the edited copies, and I always shove stuff in the recycle bin and check the contents before deleting it.
Viruses? To be honest I haven't encountered any virus which actively destroys your data. When friends get viruses, I am able to recover their data for them. I personally haven't gotten a virus for years.
So, I'm lazy. I'll get an eSATA hard drive sometime in the near future to do a proper backup, when I get the money