I was not motivated to go to graduate school for the money (despite this earlier post) or because of the state of the job market. I wanted to go because I love physics and knew that in order to "be a physicist" studying the types of physics I was interested in I would need to get a PhD.
However, this plot made by PhD Comics gathered from from the Bureau of Labor and Statistics suggests that the state of the job market may be a major factor determining whether people decide to enter grad school or not. As is noted, fluctuations in grad school enrollment in science and engineering and fluctuations in unemployment are strongly correlated. When unemployment goes up, more people start going to grad school.