According to Yahoo Sports after speaking with Greg Nelson, Lesnar's head trainer:
“I’m not sure exactly what it is he has, mono or whatever, but I know that he was sick and couldn’t get a good, hard workout in,” Nelson said. “There’s a big difference between making an appearance and smiling and waving at a football game and going out and doing what he has to do in his training.”
Some folk had their own conspiracies about why Brock Lesnar pulled out of the UFC 106 match up, citing his appearance at a recent Minnesota Viking's football game. However, I have to agree with Nelson in that there's a big difference in training and blowing into a bullhorn for like 3 seconds. If Lesnar can't go for UFC 108, it's likely that his fight with Carwin will be pushed back until UFC 109, which takes place in Las Vegas.