Just stumbled on planb when browsing on the ActiveMq site.
To quote the site :
"Plan/B is a network aware file backup system specifically designed to make the backup of files from multiple machines to a variety of devices simple."
This is exactly the concept I had in mind as a backup system for myself and since I couldn't find one, I was planning on writing a javabased backup system myself.
As far as I can see it is free, but it is not clear if it will stay that way. The one thing that is missing is some kind of API to write extensions (although I might just have missed that).