Dare, Brent, Kevin, myself and others have been discussing a standard to exchange subscriptions and item-states between aggregators. A standard like this would not only allow you to for instance sync up different SharpReader instances on different PCs (say one at work and the other at home), but would also work across aggregators so you'd even be able to for instance sync up NetNewsWire at home with RSS Bandit at work.
Dare just posted the RFC for this proposed standard. Please post any comments you may have on his weblog.