The explanation that Flynn describes in his book, What is Intelligence? is basically that modern culture gives us extra practice in many of the subskills that require a lot of intelligence. That, however, doesn't increase intelligence itself - it only makes us better at doing tasks that require those subskills.
This doesn't mean that IQ tests would have lost their value, either - if, say, everyone in the population ends up exercising an additional five hours per week, then everyone's athletic ability does go up, but it's still the ones who were the most athletically talented in the beginning who end up having the best results. The same principle applies for IQ: "general intelligence" + "domain-specific talent" + "amount of practice had" is probably a pretty good formula for figuring out how good you are at something, and if everyone gets roughly the same amount of extra practice, the tests remain a good way of distinguishing the one with the highest IQ.
In practice, the IQ tests' validity might be even better than only this would imply. The obvious question this raises is, "but does the whole population get the same amount of extra practice?". In all likelihood, the answer is no - but it's very possible that for a lot of things, those with the highest IQ get the largest amount of extra practice, since they will naturally find simple things boring and seek out the most complex things. Thus the amount of practice, itself, likely correlates with IQ.
Edit: Q&A is now closed. Thanks to everyone for participating, and thanks very much to Harpending and Cochran for their responses.
In response to Kaj's review, Henry Harpending and Gregory Cochran, the authors of the The 10,000 Year Explosion, have agreed to a Q&A session with the Less Wrong community.
If you have any questions for either Harpending or Cochran, please reply to this post with a question addressed to one or both of them. Material for questions might be derived from their blog for the book which includes stories about hunting animals in Africa with an eye towards evolutionary implications (which rose to Jennifer's attention based on Steve Sailer's prior attention).
Please do not kibitz in this Q&A... instead go to the kibitzing area to talk about the Q&A session itself. Eventually, this post will be edited to note that the process has been closed, at which time there should be no new questions.