cupholder:

I think it's more likely for the simple reason that what earlier geniuses (like von Neumann etc.) did has already been done. To me, that implies the genius bar has been raised, in absolute terms, at least in the hard sciences and math.

That could well be the case. However, it fails to explain the lack of apparent genius at lower educational stages. For example, if you look at a 30 year period in the second half of the 20th century, the standard primary and high school math programs probably didn't change dramatically during this time, and they certainly didn't become much harder. Moreover, one could find many older math teachers who worked with successive generations throughout this period -- in which the Flynn IQ increase was above 1SD in many countries. If the number of young potential von Neumanns increased drastically during this period, as it should have according to the simple normal distribution model, then the teachers should have been struck by how more and more kids find the standard math programs insultingly easy. This would be true even if these potential von Neumanns have subsequently found it impossible to make the same impact as him because all but the highest-hanging fruit is now gone.

I would bet that the standouts you're talking about would have higher average IQ, but would not actually be 'exceptionally' high, because IQ doesn't correlate that well with success.

Yes, that's basically what I meant when I speculated that IQ might be significantly informative about intellectually average and below-average people, but much less about above-average ones. Unfortunately, I think we'll have to wait for further major advances in brain science to make any conclusions beyond speculation there. Psychometrics suffers from too many complications to be of much further use in answering such questions (and the politicization of the field doesn't help either, of course).