Let me download some software and allocate how much of my drive space i'd like to help them with. The software would then intelligently use that space as their distributed backup system. Then they can focus on collection and collation with one less thing to worry about.
Let's guess that the average person can contribute some 300GB of their disk space. If IA wants to keep a minimum of 5 copies on the network (probably a safe number given how many people will be constantly dropping out), they need (20x1024x1024x5)/300 = 349,525 people contributing their disk space. That doesn't seem even close to attainable.
I originally thought that wasn't a lot of people. However, Folding@Home (which ought to be the most well known share-processing power group) only has 179,234 computers in its network.
http://bt1.archive.org/hotlist.php
Are those torrents totally not moderated?