In total, foreign born researchers accounted for nearly 35 percent of all doctorates granted in 2006 (15,947 of 45,596), and for 43 percent of the Ph.D.’s awarded in scientific and engineering fields (12,775 of 29,854). Non-citizens accounted for more than 70 percent of doctorate recipients in electrical, civil and industrial/mechanical engineering, and more than half of Ph.D. recipients in all other engineering fields, computer sciences, math and physics.
In case you were wondering: the number of Indians receiving science and engineering PhDs from US universities was 1524 in the year 2006. I don't have the corresponding figure for the number of PhDs awarded by Indian universities, but going by an earlier NSF report (see this post), this figure was 6318 in 2003. Thus, the number of Indians getting a PhD abroad is a pretty substantial fraction -- over 20 percent just for the US -- of the total number.