Just off the phone with Apple support. Their internal support documents (updated today, according to the voice on the phone) do indeed confirm that the latest 3C39 firmware does NOT fix the 10% daily drop issue, at least not for everyone. They're working on a new firmware to fix these issues. As always, no date for release indicated.
The support guy also confirmed that it's no use send the headphones for repair/replacement, as they would only be returned to me. He had no answer as to why some have these issues, while others don't. Guess we'll never know.
So I guess the unlucky ones of us that belong to Apple's 10% daily drain club will just have to wait and see.