A friend of mine was discussing how betting exchanges use escrow accounts for outstanding bets.
I did some searching and found a wikipedia entry on how it can go wrong
http://en.wikipedia.org/wiki/Sporting_Options
This led me to a link on BackAndLay
http://en.wikipedia.org/wiki/BackAndLay
http://www.backandlay.com/
This site appears quiet at the moment, but offers a free API (Betfair £2000/year for transactional service) and only 1% commission. My dreams of endless arbitrage opportunities may not be over (dream on)!