Last night, one of the guests on John Andrews's show was Jay Lehr of the Heartland Institute, talking about our national "oil addiction." He made the perfectly good point that as the price-per-barrel rises, recoverable reserves rise as well. While the days of $20 per barrel are long-gone, at $50 per barrel, nobody's running out of anything for a while, and the US and Canada suddenly have the largest reserves in the world. So in a war or some other world crisis, we should be able to get our hands on enough oil to run the military without foreign help. (In the absence of a war, the US is still hooked into the international system, where transportation of oil is relatively cheap, and prices are set at world, rather than local, markets.)
That said, it's worth remembering that recoverable reserves are also subject to political as well as economic restraints. Much of the world's exploration and recovery is now being done by government firms, which tend to be less flexible, more protective of prerogatives, and less efficient. This can't help but decrease the recoverable reserves, if only by slowing exploitation, especially of the harder-to-reach reserves, which require more infrastructure.
In fact, it's also worth pointing out that even the surface oil takes a little while to get to, while rigs are built. Large-scale shale and tar sand recovery will take a while to get up and running, and until then, we will remain vulnerable to international oil shocks.