Until mandatory helmet legislation is reviewed or removed, it is illegal to ride a bike without a helmet.
I find the quickest way to pass comment on a cyclist riding without a helmet, whether they're approaching from the opposite direction, or after you ride past them, is to loudly slap your own helmet twice, with no verbal exchange. BANG BANG. It's as loud as a handclap and gets the message across - "Where's yours?"
This also applies to people riding along with their helmet dangling from their bars - "It goes on your head, mate."
I haven't heard any good 'well, it's like, my head, man' rebuttals lately. Does anyone have one?
(this discussion inspired by the recent helmet info posted.)