There's an excellent article from the Economist on 'peak oil' reprinted at the Ecological Economics Blog.
The gist is that the peak is coming, but not here yet, and that it should be a fairly slow process rather than a crash: oil will become gradually more and more expensive as existing reserves slowly decline and become more expensive to extract. This means that adjustments, while not being easy, should be smooth: peak oil shouldn't create economic chaos.
What are those adjustments? As the price of oil rises, new, more expensive oil sources (eg, tar sands) will become economic, as will alternative fuel sources such as biofuels. On the demand side, more energy efficient technologies and practices will cushion some of the blow.