Automatically backing up uploaded files?
I have not been in the situation before, if I were in your shoes I would take a look at the CDN module.

In File Conveyor mode (ie, a push model), it will automagically copy files up to your CDN of choice.  A side effect of this is the fact that the files will now live in two places.  You can go with one of the commercial CDNs, or you can set up your own with another server.