|MadSci Network: Chemistry|
The article you are looking for is available at http://www.sonic.net/melissk/msg.html
It is an Associate Press article, and describes claims by scientists that Monosodium glutamate (MSG) triggers receptors in our taste buds differently than any other material. I think it is a bit of a stretch to call this a "Fifth" type of taste, but you can read the article yourself and decide. Also, keep in mind that this is a media article, not one taken from a peer-reviewed journal. Your best bet would be to try to track down the original source.
Try the links in the MadSci Library for more information on Chemistry.