Great post about em and en dashes over at Indies Unlimited.
Punctuation can be a mite confusing at times and I'm always striving to get it right. That's why I have someone copy edit my work, because I'm not always great on comma's and simple things like colon versus semi-colon.
Anyways, this is a very good post on dashes, ellipsis, hypen, when and where to use them. Check it out.