I've checked this for several institutions, all physics departments, for 2012.
Harvard: 36/64 = 56%
Stanford: 42/81 = 52%
MIT: 34/85 = 40%
Berkeley: 42/117 = 36%
Princeton: 21/64 = 33%
Information for Caltech is not available, but Caltech's website says that "for example," 70 may be admitted and 30 may choose to enroll, which is a rate of about 40%. However, looking at the available information, the actual entering class size for Caltech seems to have been consistently less than 30 for the last several years, probably with a mean closer to 20. Can't really tell exactly how many admissions they offer.
Granted, this is only one year's data, and there is likely to be a lot of fluctuations. For example, one year Caltech had an enrollment of 27, another year only 19, and while 8 is not such a large number, if it is out of 70 admissions then that is an 11% point difference. Still, the disparity between Harvard and Princeton seems quite strange, unless last year Harvard did particularly well and Princeton particularly bad.