I have since discovered that the necklace that she wore for the cover shoot is Fenton by Dana Lorenz. The neckace is made of gunmetal, crystal, brass and hand dyed silk. It is part of the Fenton Fall and Winter 2009 Collection.
As with some of the other pieces that the first lady has worn, this necklace can be purchased at Ikram in Chicago. It is also available at danalorenz.com