I did a back-of-the-R-session guesstimate before I posted and got a two-tailed p-value of roughly 0.1, so not significant by the usual standard, but I figured that was suggestive enough.

Doing it properly, I should really compare the PhD holders' IQ to the IQ of the non-PhD holders (so the samples are disjoint). Of the survey responses that reported an IQ score and an age of 29+, 13 were from people with PhDs (mean IQ 146.5, SD 14.8) and 135 were from people without (mean IQ 139.3, SD 14.3). Doing a t-test I get t = 1.68 with 14.2 degrees of freedom, giving p = 0.115.

It's a third of a SD and change (assuming a 15-point SD, which is the modern standard), which isn't too shabby; comparable, for example, with the IQ difference between managerial and professional workers. Much smaller than the difference between the general population and PhDs within it, though; that's around 25 points.