Yellow Avalanche Lily, a native species, has been added to the USWildflowers database (06/10/2010.)  Scientific name is Erythronium grandiflorum.  This plant also also goes by the common name Glacier Lily.  Photo below was taken Boise National Forest near Boise, Idaho on 06/08/2010.

Yellow Avalanche Lily - Erythronium grandiflorum