On a bike path, I certainly will. I rarely get to ride and listen to music... so once every 3 months when I hit the bike path, I jam to my tunes. It makes the time go by faster since I am riding alone.
On the roads, in my cycling groups, I NEVER do.
Honestly though, even without my headphones, I miss cars coming up on me. Especially when they are SPEEDING. I hear the car right when it's on me.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"