OP here. Etymotic did offer to evaluate my HF2's. Their e-mail said if they found a problem they'd replace, and if not they'd contact me. I'm not optimistic they'd find a defect.
I took my HF2's into an Apple store and tried it out with a floor iP4. Just once it seemed like the button didn't work, but after several more minutes fussing with it I was unable to reproduce the problem.
There's something giving me the impression that the issue has something to do with the quality of the connection between the 4 contacts on the plug and the iPhone. I think the problems are worse when the plug isn't 100% jammed into the hole.
Just did another test with my own iPhone 4. Really poor performance, frequently not recognizing the button presses even when I apply pressure on the connector. I think based on this I should ask Apple for a replacement.
ETA: Used some canned air to blow into the plug. Now it's testing almost perfectly. This is like the proximity issue, hard to nail down.