if it is the first time this has been brought up I just wanted you to know, and if it has already been spotted the I have told you anyway. (If you want to find it it's at the end of the tile/block area of the en_CA language file which is located in 'lang' in your minecraft.jar file).
Originally the rubies were going to be the currency item for trading with villagers. But then they changed it to emeralds, I'm assuming, because there is already a red ore (redstone) in the game.
There is no need for rubies now though, so they are unlikely to be added.
There is a brief mention of them on this wiki page http://www.minecraftwiki.net/wiki/Ruby#History. I believe the ruby sprite is still in the image file too.