The Cathay have always been a part of Warhammer lore. They are a human race, and have made several appearances in warhammer as dogs of war units (ninja monks!), but have never had their own army list. As far as I know, there will never be an army book for Cathay, although the idea has come up in many rumour sections on message boards.
A great source for more information on Cathay is the Warhammer Role-playing game (has recently been reprinted). It is in essence an amalgamation of many far-eastern cultures and mythologies. I have seen several people use the empire army list to represent a Cathayan army to great effect (karl-fran on imperial dragon = heavily converted Cathayan emporer on chinese dragon, rocket batteries, lots of crossbows and lots of spearmen)