I'd guestimate half the commuter and casual cyclists I see are wearing headphones or earbuds these days -- it's legal in Washington State, the ban on headphones applies only to motor vehicles, and only to headphones that block ambient sound.

And of course not all headphone use is the same. I ride with a bluetooth headset that's specifically designed not to block ambient sound, and I keep my music at a low enough volume to hear my own drivetrain, so I have no trouble hearing overtaking traffic, except for a few miles of my daily commute that are on I-90, where freeway noise drowns out any individual vehicle.

I actually believe helmet straps are more of a hearing obstacle for many cyclists than reasonable headphone use -- have you ever noticed just how much wind noise a typical helmet creates at speeds over 10-15mph?