The various types of non-toxic shot approved for waterfowl are listed here, including tungsten:

http://migratorybirds.fws.gov/issues/nontoxic_shot/nontoxic.htm

The article I read regarding tungsten shot was interesting and I wish I could remember the writer. His opinion was that the FWS mandating steel shot was a blessing in disguise for waterfowlers. It forced ammo companies to develop new and improved shot wads, buffers, and propellants in order to make steel shot perform at an acceptable level. Now that bismuth and tungsten have been developed, which are much denser than steel (although still not as dense as lead), the result is a much better overall package than what was available 20 years ago with the old-fashioned lead shot.

Regards, Vince