Although this was kind of asked in this question, it was as a side-question that wasn't really answered.
I've heard people say you should put salt in coffee to reduce bitterness. Does this actually work? If so, why?
The fundamental science seems to be: Sodium salts suppress the perception of bitterness. It specifically mentions caffeine, which is one of the bitter components of coffee. This About article lists a few more references to the same thing: The sodium ion works by "interfering with the transduction mechanism of [bitterness]."
The point is, it's a change in perception; e.g., the salt is not chemically changing the bitter compounds, just interfering with the bitterness receptors in your tongue. Conversely, the article seems to suggest that the bitterness doesn't reduce the perception of saltiness; this to me suggests that your coffee will taste salty. Caveat emptor! There's a bit more discussion on the saltiness and bitterness sections of the Wikipedia article on taste.
This question from Seasoned Advice discusses the same and provides some additional links. The Good Eats article linked from that question is interesting. A search at Seasoned Advice discusses related topics of salt and bitterness.