h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

What about the arm-under-the-crook-of-the-neck school of CFODABPLO? It means the front person has to either:

Be a bit lower down than the behind person.
Be a bit smaller than the behind person.

I'm sure there's an appropriate body mass ratio to be worked out here. A front person to behind person ratio of about 1:1.3-ish, say. I guess it would also help if the behind person has got skinny arms and if the front person has, well, got a 'gappy' neck (helped with the aid of an appropriately heightened pillow).

*falls over laughing in little bits at the phrase "gappy neck"*

I'm sorry, but that sounds like a personal problem. "She was a real cute girl, but she had a gappy neck that I just couldn't live with." LOLOLOLOLOL!

I'm with you on that. I'm 6'1" and my girlfriend is 4'11". This gives us a ratio of 1:1.23. Spooning works well here, but clearly just in one direction.
I find that my arm is most comfortable under her neck, although my forearm does tend to get in the way, and even the weight of a neck in the wrong place can cut off the circulation. It usually ends up folded between my two pillows (both of which are quite thin, so my neck doesn't get bent too much.) under my head. I find that my arm tends to create it's own space in the lower pillow allowing the top pillow to lie flat across them.
Hair doesn't seem to be a problem. It can either be pulled forward out of your face to lie curled around her chin, but even this isn't necessary, as her hair is of sufficient length (Slightly longer than Shoulder length) so as to keep the ends well away from my face. This, however, creates it's own problem when your partner decides to sit up and get a drink/go to the loo. Hair tends to be caught under your own arm which can be incredibly painful if he/she sits up too quickly.
Finally, the one thing that stops us sleeping in that position is not arms or hair, but heat. It doesn't take very long for heat to build up to uncomfortably clammy levels. forcing you both to separate. What's needed here is some kind of reasonably effecient heat sink. This problem, of course, only affects living mammals, and doesn't apply if one of you is dead or a snake.

Phew! Boy! I'm glad I'm dead! I agree with the heat problem in the summer, but in winter it's perfect. We turn the heat wayyy down so snuggling is a good thing. When it's cold outside the covers and toasty in it makes for some excellent snoozing.