Skype IS Internet-based VoIP Peering and a very good example of it. Proving the point that multi-lateral (free) calling is preferred over the next best choice in a least-cost route Rich Tehrani just blogged about an Om Malik analysis of Skype's numbers in an eBay filing.
Two very powerful elements of multi-lateral VoIP Peering, compounding viral growth and user cost savings realized from it, were captured in this comment by Rich,
"The problem as you might imagine is Skype-to-Skype minutes are growing much faster than SkypeOut and this only makes sense when you consider broadband is spreading and computers are getting much cheaper. In fact you can potentially justify a netbook from your telephone savings of a month or less in some countries/situations."
The "problem" being referred to is that the growth of multi-lateral is a hit to Skype revenue. The benefit to the user is reflected in the ability to buy a netbook (maybe notebook computer?) with the savings. This is the way it works for huge corporations too. Multi-lateral on-net callng cuts costs and can save enough to keep jobs, buy new gear, finance the business, year-end bonuses, whatever...