White Mariposa Lily, a native species, has been added to the USWildflowers database (06/09/2010.)  Scientific name is Calochortus eurycarpus.  This plant also also goes by the common name Star Tulip.  Photo below was taken at Deer Flat National Wildlife Refuge near Nampa, Idaho.

White Mariposa Lily - Calochortus eurycarpus