Went to a concert last Thursday, and was in the mosh pit when near the end someone's shoulder went straight into my chest. Since, I've had a soreness there - it hurts to breath, and feels bruised with nothing visible, and certain movement hurts it.
The pain isn't severe though - even at its worst I can deal with it, and at times there is no pain at all, and from what I've heard even a less severe bruised rib is particularly painful, but I don't know what else it could be.
-- Joyrock Fresh from my first justified ban. Ever!
I would recommend avoiding activities like running where you would be taking a lot of deep breaths, as well as trying to keep from aggravating the injury. Pain from normal breathing should go away in a few days, but the injury will probably last a few weeks. It usually heals on its own.