Colleges such as MIT, Caltech, and Harvey Mudd that really want to measure applicants' knowledge of math through pre-calculus require the SAT math subject test. Some selective colleges require two SAT subject tests, but most of those do not require that one of the subject tests be in math.
The list of colleges requiring subject tests
http://collegeapps.about.com/od/sat/a/sat-subject-test-required.htm is not long. Apparently, most colleges have concluded that the loss in applicants caused by requiring another test outweighs the benefit in assessing the preparation of applicants.
