The National Science Foundation has published two reports on American research and the decline of journals publishing it. The reports show that beginning in 1992 journals began to publish less American based research with a corresponding rise in research from Europe and Asia being published. In 1992, the share of American research published in journals was 37 percent and in 2003 it was 30 percent. The reports give a number of possible reasons for the decline, including the increase in scientific research being performed in Europe and Asia as well as more international collaboration on research in all fields.
Both reports are interesting and worth a read along with an article about them in Inside Higher Education. A third report on the topic is planned.