Is head-butting for real?
First of all, John, it's not usually called "skull-crashing" the practice you're referring to is typically known as head-butting. And yes, sometimes people really do engage in it.
The most widely witnessed real-life head-butt in history was almost certainly the one given by French soccer star Zinedine Zidane to Italy's Marco Materazzi during the 2006 World Cup finals. Materazzi later admitted he had provoked Zidane, and both players were disciplined. In a 1992 New York Times article, Morgan Gendel reported that soccer players in the northern British Isles called the maneuver "sticking the nut," and that it was a "street-fighting move popular mostly in working-class sections of Britain and Ireland."
Of course, head-butting is unwelcome in most sports, even those directly involving combat. A head-butt motion shows up sometimes in the traditional sequences of karate moves known as kata (there's one at about 1:21 in this clip of the kata called Gojushiho Dai), but head-butting an opponent is off-limits in competition.
Other martial arts do allow the head-butt (Burmese boxing, for one), but still it doesn't happen a lot, and many experts discourage it. Why? Because, as you suspected, a head-butt can knock out the bestower as easily as the recipient, especially if it's directed at the sturdier parts of the head. Ben Ryder of the Newcastle University Shotokan Karate Club suggests a target limited to "a circle just covering the eyes, top lip and the cheek bones," as there's lots of soft tissue there. He also suggests aiming for the ear if you're striking from the side, for the jaw if striking from below. Zidane knocked Materazzi down with a head-butt to the chest.
Striking elsewhere, martial-arts experts are quick to warn, can cause injuries. Writing in the online martial-arts publication Uechi-Ryu Journal, Scott Taylor and Gaz Morgan explain how to counter an attack from behind with a backward head-butt, but they clearly paint it as a high-stakes gambit requiring solid technique: "When dealing with the subject of survival there may be no other option for you. If done correctly though you can connect with your attackers [sic] nose and avoid substantial self injury." In this video martial arts instructor Richard Grannon stresses the importance of accuracy, noting that the Red Eye-style forehead-to-forehead strike is "not good for anybody."
Self-defense expert W. Hock Hochheim, in an article on head-butting at his Web site, quotes neurologists who warn against concussions or worse and maintains that "some mighty big name martial artists have been carried out of the bar after knocking themselves out doing a head butt." Even if your head-butt doesn't actually leave you unconscious, you could still wind up momentarily dazed, which in the middle of a fight is never good.
In sum, authorities agree head-butting can be a useful technique if done with skill, but they suggest you save it for special occasions namely, when it's basically the only move you've got left.
It's a different story, of course, in Hollywood, where consequence-free head-butting has been big since around 1990. At least in Red Eye, the bad guy's forehead bleeds a little after his successful head-butt; in most movies the person doing the butting is uninjured while his victim is knocked cold. Here at the Straight Dope Message Board we were unable to identify a mainstream film depicting a head-butt before 1980, when the move turned up in both The Road Warrior, starring Mel Gibson, and Roadie, starring Meat Loaf. In the Times article mentioned above, Gendel pointed to Gibson's use of the head-butt in 1987's Lethal Weapon as the beginning of the fad in U.S. cinema. After that, it soon showed up in Black Rain (1989), Internal Affairs, Kindergarten Cop, Dances With Wolves (all 1990), One Good Cop, The Hard Way, Hook, Star Trek VI, The Commitments (all 1991), and Batman Returns (1992).
Mike Figgis, who directed Internal Affairs, has said the key to a good movie head-butt is the sound effect; the one in his film uses the sound of "wood on hard cabbage."
And sometimes the line between art and life gets blurry. Veteran onscreen head-butter Gérard Depardieu allegedly head-butted an Italian photographer in 2005; in June he was ordered to appear in court in Florence.