I'm giving the edge to Lesnar in this one. I've watched Lesnar's last two UFC fights this last week, he just beat the shit out of Heath Herring and was caught with a submission in his fight w/ Mir. Heath Herring, he just threw around on the ground and kneed the ever living shit out of him.
Oddly enough, I've never watched Couture fight, I only know him by his reputation.
He, Lesnar, is going to be VERY vulnerable to any submission other than a choke. I don't see anybody getting and keeping an arm around that huge neck of his.
If Couture ends up on his back, he is going to get hurt. Go back and watch the Lesnar/Herring fight. Once Lesnar has you down he has the strength and agility to keep you down. The one major thing he, Lesnar, is missing is the ability to finish an opponent as illustrated in the Herring fight. If Couture can keep alive past the first two rounds, he has a better than even chance to win it by submission after that.
Here's my call - Lesnar in 2nd TKO.