It's not often I read a music review/survey and know all the major players, but "ambient Americana" is probably where a good chunk of my music resides, as well.
Steve Roach: Lots of his stuff, but more purely ambient than Americana. "Dust to Dust" is an exception, and exceptionally good.
The first two albums by the Grails,"The Burden of Hope," and "Redlight." The Grails later albums are much denser, heavier, and move away from the more Americana-ish first discs.
Daniel Lanois "Belladonna" should be required listening, and I'm surprised Diliberto didn't list it.
Earth's "Hex" and A Small Good Things, "Slim Westerns Vol. 1 & II" are both excellent soundtracks to dark Westerns that never were. "Hex" is particularly awesome.
Finally, whenever there's mention of Morricone, I have to bring up the sadly out of print "For A Few Guitars More," which is all surf covers of Morricone themes. Absolutely one of my desert island albums.