That is ideal but not especially easy. Hashing as they are doing it requires several factors: what you need to compute is not handed out by a central authority

That is ideal but not especially easy. Hashing as they are doing it requires several factors: what you need to compute is not handed out by a central authority, it is hard to compute, and is easily provable once computed. folding@home and others do not usually fit 1 and 3. However, there have been some attempts, see Curecoin and Primecoin to a lesser extent.