I bought up the issue of salt vs bitter in my Salt, Salmon and grapefruit! post. I wasn't sure what was happening to the bitter flavor when you add salt to food. I had a bit of a hunt around and couldn't find a reason why . So I emailed a question to cookingissues, and Dave Arnold answered the question on his show.
Basically the salt alters the way your taste receptors interrupt bitterness (maybe). Go here to listen to the radio show or subscribe to the radio show via podcast.