Honestly, I don't think LeBron would last a full practice in the NFL, let alone a season. Several of my college teammates played with/against him in high school, and they all said he was completely overrated in football solely due to his publicity in basketball. There are a ton of amazing players in Ohio, and there's no way he deserved to be All-State. And the plays he did make were just because he was so much bigger than everyone. He rarely went over the middle, and was used primarily as a down field threat.
I get so sick of basketball announcers talking about how great this player or that player would be in the NFL. The NFL is approximately 50 million times more physical than the NBA. Most NFL players are some of the toughest people alive. LeBron would certainly be tough to defend in jump ball situations, but there's no way he'd be able to take a hit from Troy Polomalu, Ray Lewis, etc on a regular basis.
And for people who say he could play tight end, there's a little thing called BLOCKING. Good luck putting a body on Julius Peppers and Dwight Freeney every Sunday.
I know a current trainer for an NFL team, and he used to work for an NBA team. I asked him about the difference in pain tolerance between NBA players and NFL players, and he started cracking up and said it wasn't even close.
IMO, the only guys in the NBA that would have a realistic shot are the blazing fast point guards (Rondo, Rose, etc) who could possibly play corner. Those guys can fly, and you can get by as a corner in the NFL without being overly physical. Also, you won't be the one receiving hits very often as a corner.