So, that Graham over at We Ain't Got No History? He's a smart dude. He used to be an engineer, then he dove full-on into baseball's statistical side - in addition to many other things, he co-invented a pretty sweet baseball statistic, and now he's turning his stat-oriented mind to soccer and its nascent statistical analysis efforts. He recently wrote a piece on the fallacy that is the possession statistic - what it is, what it purports to be, and why it isn't really a very accurate thing at all in its current incarnation. You should go read it. And really, when it comes down to it, you should always read Graham's stuff, because it's really really good and he's really really smart.
If you're at all interested in advancing the statistical analysis aspects of soccer, this is awesome stuff.
UPDATE 4/23: Now, Graham has developed his own method for calculating possession. Seriously, people, this is great stuff. Go check it out.