<p>^I can answer that for one year (the year I worked in the admissions office, 2006, for the class of 2010). I assume the data is fairly consistent from year to year.</p>
<p>That year, the most frequent non-MIT choices were Harvard, Stanford, Princeton, Yale, and Caltech (in that order). Those schools accounted for about 60% of students who chose to go elsewhere, and Ivy League schools other than HYP account for another 10%. State schools only account for 10%, which is really surprising when you consider the excellent engineering programs at, e.g., Berkeley and UMich.</p>