...I take it that copying the files to disk is not an option?
Actually I do burn My Documents and the folders on my desktop to a DVD once in a while, minus the music, videos, and photos (those are the files I would mostly upload to a web service). I even have a recent DVD in my safe deposit box at the bank in case of fire, meteor, or errant satellite.
I plan to buy an external HD, but it will be a few months before I can accumulate the cash. The web services seem convenient for a procrastinator like myself, and I'm just curious about their trustworthiness.