Yet another news article scaring the pants off ordinary people by suggesting that Notional = Value, and that Collateral = Zero. It's a shame, the comments show how taken-in people are when the underlying facts aren't presented properly. See the other side of the story on this site here (How Big is OTC Really).
I came across this long PDF from Deutsche Bank which explains a great deal about the evolution of clearing, across OTC rates & credit, and into FX & futures. There have been some developments since this was published, but overall it's a useful document. The document originates from http://dbclear.com/