The Washington Post has a very good Op-Ed piece about how complexity theory explains the financial crisis. Finally. I've been thinking about how complexity theory could help us all makes sense of this mess. This is a good first step.
They key takeaway is that companies evaluated risks sui generis, as individual phenomenon, without analyzing connections to the system as a whole as thoroughly as necessary. Given normal conditions, the computer models predict that financial instrument X has Y percentage of going wrong. However, if conditions change very dramatically, beyond the parameters of the model, the chance that something will do seriously wrong also change dramatically.
What's key to understand is that the introduction of these computer models were themselves a change in the conditions of the markets.