Okay, I've probably done enough laughing from the sidelines, so let's critically examine the scenario before this thread gets any more absurd (hello Ali vs Bruce Lee post).
Navy SEALs do not train all day long to beat people in unarmed combat. They train all day long to shoot terrrrists and rescue hostages and maintain their conditioning, after having proven their elite mental toughness and physical conditioning to get in. That mental toughness and conditioning is the main relevant aspect of being a Navy SEAL in an unarmed fight against a professional MMA fighter. Some of them will have further martial arts or combat sports training, but most of their time is spent training for scenarios pertaining to their job description.
Brock Lesnar is a former NCAA division 1 wrestling champion and current MMA fighter, top 10 at heavyweight. Ignoring his recent health problems, he walks around at around 300 pounds with very low body fat and has very high physical strength, along with good cardio for his size. His job, at which is excels, is to beat very skilled and strong people in unarmed combat under minimal rules. He accomplishes this by taking his opponents to the ground using his wrestling, controlling them, and punching them repeatedly with his raw physical strength.
Navy SEALs are not Arnold Schwarzenegger from Commando. Like any special forces unit, they're composed of athletic guys with good power to weight ratios who can carry a lot of gear while running a long ass time. They are typically going to be giving up a 1.5-2 to 1 physical strength advantage to Brock and around 100lbs in size. Size and strength (assuming conditioning is also present) matter a great deal in unarmed combat.
This nonsense about "Navy SEALs are trained to kill, not to just win under a set of rules" is absurd. You become proficient at fighting in large part by training against skilled fully resisting opponents. You cannot use the "magical elbow of death" against your sparring partner. Nor can you use the "magical hollywood neck snap" against your sparring partner. MMA fighters train what they *can* use, thousands and thousands of times against fully resisting opponents, to the point where they are extremely good at their tools of choice. Someone learning the five fingers of death from their sensei and doing it in the air a few times with a kiai doesn't mean shit. And Navy SEALs aren't going around engaging terrrrists in unarmed combat every night, if ever.
http://en.wikipedia.org/wiki/United_States_Army_Combatives_School Hey wait, why is the US Army using MMA as the basis for its unarmed combat training and not the MAGICAL ELBOW OF DEATH? What a shame, they could be killing people in a single hit!
Brock wins every time.