I won't ride with anyone on a group ride who is using earbuds. This is group riding 101 as far as I'm concerned and I have no problem with telling them to turn it off. It's not only dangerous, it's rude and anti-social.
It's probably as bad idea to listen to music on a bike under any circumstance. At least I wouldn't do it personally. Then again, I also think it's a bad idea to run with music. I approach runners with earbuds very carefully on my bike: they are essentially dead to the world in my experience and don't seem to appreciate that they need to pay attention.