Lazyweb request for peer-to-peer backup system

By jove, I think David Weinberger has spun out yet another brilliant idea in P2P backup

I think I'm missing something obvious, but why can't I find a p2p backup system that lets me and a designated buddy swap storage space? I'll give my pal, say, 5GB of storage on my computer if she'll give me 5GB on hers. My computer is pretty much always on, and so is my buddy's. All we need is some basic sw for letting us designate the directories we want kept up to date and for making the p2p connection. Maybe a little encryption and compression. Neither of us guarantees 24/7/365 access, multiply redundant raid arrays, or whatever, but it would help me sleep better knowing that when my house melts, the drafts of that unfinished awful novel will survive.

Does this software — preferably free and open source — exist and I've just missed it? If it doesn't, have I missed why this is a bad idea?