Well beyond the obvious Sigur Ros suggestion, I would go with:

1. Iron and Wine - Creek Drank the Cradle
2. My Bloody Valentine - Loveless
3. Beach House - Devotion
4. Radiohead - Kid A
5. Air France - No Way Down
6. Beck - Sea Change
7. Beirut - The Flying Cup Club
8. Culture Reject - Culture Reject
9. The Garden State Soundtrack
10. Van Morrison - Astral Weeks

So Boring You'll Fall Asleep:
1. Panda Bear - Person Pitch
2. Andrew Bird - Noble Beast
3. Republic Tigers - Keep Color

I love falling asleep to mellow instrumental music.

I know it has been said a few times, but so far Eluvium is my all time favorite, check out their albums Copia and Lambent Material (check out the last song on Lambent Material before including it in a playlist, it could wake you up)

Honorable mentions...
Thom Brennan - "The Mist" (ambient)
David Schombert - "Quiet Life Vol. 2" (new age)
Biosphere - "Substrata" (ambient)
Patrick O' Hearn - "Indigo" or "Road to Eldorado" (new age)
Harold Budd & Brian Eno - "Agua" or "The Pearl"(piano & ambient)
Helen Jane Long - "Porcelain" (piano & violin)
Miles Davis

Definitely Grouper's Dragging A Dead Deer Up A Hill. Beach House's Devotion and Fight Bite's Emerald Eyes too. I like mellowing out to Marissa Nadler's Song 3: Bird On The Water, but you have to listen to the narrative of her lyrics too much to fall asleep to it.

William Basinski's Silent Night album.

Circulatory System's Circulatory System.
Hands down the best thing to fall asleep to.

Anything by Brian Eno will do the trick