I go back and forth on this one. On one hand Khan is "chinny" and has lost to Garcia, Prescott and Peterson, all of whom are a full level (or three!) less than Floyd. On the other hand, I think he has the following things going for him:
1) speedy boxers will bother Floyd more than straight ahead brawlers, and Khan is fast/talented. Style wise, this is a bad match up for Floyd in my opinion, especially a 37 year old Floyd.
2) while Floyd can punch, I don't think he hits as hard as Garcia or Prescott, and I don't see him knocking Khan out with one shot. This isn't to say that Khan will be able to neglect his defense and just walk in on Floyd, just saying I don't see Floyd's shots having the same affect as those other guys' shots.
3) Khan has looked horrible in fights where he overlooks the opponent. That won't be the case here at all. If anything, I expect Khan to be up for this fight and be sharper than we've seen him in years, or maybe ever. Khan seemed to take zab seriously and I feel that he was very sharp in that fight.

I feel that Khan will give Floyd problems, perhaps more than Maidana did during their first fight. Chin, stamina and ability to adapt are my biggest questions/concerns with Khan beating Floyd, and I think all three will play a factor in the outcome. I see Khan giving Floyd nightmares through the first half of the fight. Floyd's stamina advantage will allow him to start applying the pressure and coming out ahead in more and more if the exchanges. I look for Floyd go buzz him late and score at least one knockdown. I would bet on Floyd winning a close decision thanks to a knockdown late, and having to fight hard and come from behind to do it. I wouldn't be overly surprised if Floyd got the late stoppage either though...