A quick search at www.google.com on "physics of human punch" yielded a page at the Physics of Sports course website at the University of Washington in Seattle, WA. Professor Tom Steiger calculates a Karate punch to have about 150 Joules of energy. To convert Joules to Foot Pounds you multiply by about 0.735. So your answer is about 110 foot pounds. Maybe that would be for a Karate expert, so you might want to put it at 80 or 90 for an average person.

There's an article in the April 1979 issue of Scientific American, by a physics instructor and karateka, which covers this.

According to the article, the speed of a top karateka's fist is between 10 and 15 m/s. This measurement was made using high-speed film, and confirmed using a strobe light.

We can calculate the energy of the blow using the kinetic energy = 1/2 mv^2 calculation. The mass of the fist plus forearm is 2-4 kg, so the total energy is in the range 100-450 Joules, or 70-330 foot-lbs.

The difference between a boxer's punch and a karateka's punch is that the boxer is trying to maximize the transfer of momentum to his opponent. So the boxer will put more of his body weight behind the punch and follow through, whereas the karateka will snap back his fist after impact. The energy of the karate punch is delivered more quickly, and is more likely to do local damage, rather than knocking the opponent backwards.

How much energy is there in a boxer's punch? Imagine an uppercut that lifts the opponent off the ground. The energy required to do this is mgh, where m is the mass of the opponent, g the acceleration due to gravity, and h the height to which the opponent is lifted. A realistic value for h might be 0.1 m; if we take the opponent's weight to be 100 kg, this requires about 1000 J of energy (or 700 foot-pounds). So an upper limit of 1000 foot-pounds is not out of the question.

That sounds too low (although I'm not sure I beleive the 1000 foot pounds a university team claimed for Rocky Marciano). If you watch high-grade Karateka hitting punch bags it quickly becomes pretty obvious that a boxer generating some leverage off the floor generates a lot more force. The rules of boxing don't prohibit karate style punches - if they generated power people would use them. Someone wearing kevlar body armour and a trauma pad can take a 9mm pistol round with no difficulty, despite full transfer of force. That's about 400 ft lbs.

An interesting question. The last answer was probably correct. I read that in 1955, Rocky Marciano had his punch measured at a USA military installation (as alluded to earlier) and I assume the test was conducted on a ballistic pendulum. The result was actually 925 foot pounds for Marciano's overhand right -- the same punch that nearly tore Jersey Joe Walcott's head off in their 1952 championship match. The thing that made the feat even more amazing, was that Marciano was wearing a regulation 12 oz. boxing glove at the time of the test.