Craig,
Without having read Erickson's book, let me suggest that readers treat his claims--not just of having debunked MPT, but also in favor of active management--with a considerable grain of salt.
One of the great enduring problems of back-testing an investment strategy is that it generally doesn't continue working in the future. Not can't, just generally doesn't. If Erickson's "sector rotation" strategy--which is so simple and so obvious, yet so effective!--continues to work in the future, my hat is off to him. But I doubt it.
More importantly, nobody has ever said that active investment does not generally produce better risk-adjusted returns than passive investment--just that active investment in the real world does not generally produce better risk-adjusted NET returns. It's not that active managers don't have any expertise in identifying better-performing sectors, better-performing economies, better-performing individual stocks, etc.--just that investors don't generally benefit from such expertise, either because it requires costly data collection or because the investment managers soak up the additional returns in the form of management fees. (Look at hedge funds and private equity, which unquestionably can create what look like higher risk-adjusted returns using a combination of investment expertise, excessive leverage, and under-reporting of risk, but which equally unquestionably take more than all the actively produced risk-adjusted returns in the form of fees and expenses, leaving investors worse off.)
If Erickson's investment expertise is as awesome as he seems to think, then he'll be the next Warren Buffett--unless his technique is as simple and obvious as he seems to think, in which case he won't. I very much doubt that this book will be remembered as a monumental contribution to investment practice.