Interesting article on a substance called 'miraculin', found in a berry called "miracle berry", which causes sour flavors to taste sweet for a short time:
http://people.howstuffworks.com/flavor-tripping.htm
There's also an interesting part in the article on a company in the 70's that used miraculin in diabetic products, and was about to branch out into food products, until their office was broken into and the FDA suddenly decided to reclassify the substance in such a way that the company couldn't afford to have it tested and approved.