Right now, when I do listen to music while riding, I use a pair of Koss plugs that were like 15 bucks. They are a lot like a foam ear plug, but have a tube down the middle the music comes through. Actually have decent bass for an earplug.
Here's the link for it.
Also, I have the scala rider team set, which is mostly for bluetooth (GPS, cell phone wireless, passenger) that has speakers a lot like the ones that Tigger showed. The teamset comes with two (notice team) sets, one main one and a passenger one. Well the speakers on the passenger one were torn off when I bought it and I hadn't gotten around to replacing it yet. I was looking at their website and its 30 bucks for the regular one and 50 for one that has a mp3 input jack in it that I'll probably end up getting soon. And if I get a phone call, it just mutes the music till its over.