I was on this "mystery lean condition" bandwagon recently, and here's what I discovered: When checking my PCV valve, I noticed that the secondary vac hose that leads to the IAC was twisted and closed off. I probably didn't re-install it correctly after my recent spark plug change. Replaced the hose, cleared the code, all good for the last two weeks and even passed Cali smog w/ flying colors.

Moral: give your engine bay a good look over before replacing parts. It may be something easy, cheap, and obvious.