Science 11 July 2008: A new study has found that the most likely undergraduate alma mater for those who earned a Ph.D. in 2006 from a U.S. university was … Tsinghua University. Peking University, its neighbor in the Chinese capital, ranks second. Between 2004 and 2006, those two schools overtook the University of California, Berkeley, as the most fertile training ground for U.S. Ph.D.s (see graph). South Korea's Seoul National University occupies fourth place behind Berkeley, followed by Cornell University and the University of Michigan, Ann Arbor.
But, see Brain drain slowdown. (Also: IIT uber alles? :-)
Of course, the schools listed above all have large enrollments. If you normalize by undergraduate population the top school is Caltech. (Almost 50 percent of Caltech undergrads go on to earn a doctorate.)
I was at Tsinghua in December. Surprisingly, they don't heat the buildings in winter -- even the school of management had freezing cold hallways (individual offices and classrooms are heated). It seems brutal, but it's very green! Apparently 40 percent of the US energy budget goes to heating and cooling buildings. Just look at your winter heating bill and you can understand why central heating is a luxury in developing countries (energy is a tradeable).