I have an old Minolta 28mm f2.5 MC W. Rokkor SI lens that apparently was manufactured with rare earth elements (lanthanum & thorium). The glass has yellowed and I've read (in the following link) that exposing the lens to UV light can help remove that. I haven't tried to remove the yellowing, but, this is simply a fantastic lens with b&w film. One of my favorites.

There is some interesting info here regarding radioactive Kodak lenses, among others: http://www.camerapedia.org/wiki/Radioactive .

Marc