I think I heard on TSN one time that if you do end up falling headfirst towards the boards on your belly, to keep your head up. It would do more damage if you were to bend your neck down...
Yeah, i heard the exact same thing on a video of dirty hits months ago. It's the first one in the vid:
EDIT: The embedding wasn't working.
http://www.youtube.com/watch?v=RaGd_jDkzJY
I'm not sure at all, so please don't take my word for it as i have no medical background, haha, but i think the neck is stronger along the back of the spine, so that if you were to tuck your head in towards your chest (bend your head down) you're more likely to break your neck as there's little resistance to it, because the back of your head will hit the boards and the force pushes your head further towards your chest. Sorry to be so graphic!
I also think it's got something to do with moments. If you bend a stick perpendicular to it's length, it'll break easily, but if you apply force along the length of it, there's little chance it will experience any damage. In this case, the stick is your spine, so experiencing a force along the length of it (by keeping your head up) is much more preferable!
Going back to the first point though, I think it works along the same principal of why you don't tuck your thumb underneath your fingers when you throw a punch - because the thumb is designed to bend that way. So when you do it, the impact forces your fist into your thumb and it bends into the gap left by your muscles and palm, further than it's supposed to go, causing a breakage or other painful injury.
If your thumb is on the outside, your fingers brace the impact. It's all that business of equal and opposite forces and such.
So logically i would say if you were knocked on your back, you'd want to either keep your head straight or tilt it back slightly, but never curl your head towards your chest, for the same reasons mentioned above.
I'd still like to hear a proper professional reasoning for it though. And i kind of wish someone at our club would tell people this stuff, because it's pretty important!