Most of you guys have probably seen this already, but it does prove that a stock market can be taken down for a short while now by viruses.
http://www.theregister.co.uk/2006/02...tock_exchange/
Admittedly it is a long way from taking out the world economy, but it must have cost some people a lot of money. And it doesn't look like that was the intended effect. If somebody set out to do this then you have to wonder if they could manage it.