Take a P2P system which splits up files and distributes the contents across the net already. Put an encryption front-end onto it so that any files written into it are encrypted. Then put a front-end onto all of that which allows you to "mount" the whole thing as a virtual drive.
Anyone who wants to have "X"Gb of data storage on the system needs only buy an "X"Gb drive and make it available to the system. In exchange for making "X" Gb available, you would have your files written (in encrypted form) to "the net" and automatically spread out across the Internet. If you want those files mirrored, then you can make "2X"Gb available in exchange for "1X"Gb of space, mirrored.
Just doesn't seem as though it would be that hard to do given all of the P2P software out there which already does a lot of this. I just, unfortunately, do not have the time to do it. Sigh.