Large-Flowered Trillium, a native species, has been added to the USWildflowers database (08/27/2010.)  Scientific name is Trillium grandiflorum.  This plant also is known by the common names Great White Trillium and White Trillium.  Photo below was taken near Tray Mountain in White County, Georgia on May 1, 2010.

Large-Flowered Trillium, White Trillium – Trillium grandiflorum