CNN Money has a short post about the history of the minimum wage. The chart shows real dollars, and the adjusted for inflation amount. As you can see, the purchasing power of the minimum wage is down quite a bit from the 1968 high.
Not only does the US underpay when compared to itself, we also underpay compared to many other advanced nations. Foreign Policy has an excellent article showing that the US underpays its lowest workers when compared to other advanced nations.