Here's an interesting analysis. Basically, Lebron played equally poorly against Dallas in the regular season too and Berri attributes his struggles to the Mavs' defense rather than some deep-seated psychological problem with Lebron.
http://arturogalletti.wordpress.com/2011/06/17/is-lebron-a-choke-artist-and-other-remaining-playoff-thoughts/

The analysis is sound. I think Lebron faced a situation that he didn't yet have an answer for. His game hasn't evolved to the point where he can overcome what the Mavs were doing. The Heat in general didn't have an answer. The Heat are almost all perimeter. Bosh talked about not getting enough post possessions but he's not that kind of player. He has to face up and that still plays into the hands of the Mavs D.

Lebron is going to have to work on his post game like Jordan had. It's part of the reason the Heat had trouble closing out games. The Heat are not versatile enough and any really solid defense can stop them. If they aren't scoring off of their defense causing turnovers, the Heat have a tough time scoring against set defense. The Mavs didn't have to have an all world defense to beat the Heat. That's why I still believe in the Knicks chances. If we can improve the defense our offense is already very potent. As Iman said if we can hold teams to one shot and done, we'll win. How's that for some great thinking by a Rookie. The point of Iman is to keep teams on the perimeter taking long shots with a hand in their face.

If we're ever to beat the Heat this is the direction we have to go in. Versatility is a huge part of what makes a championship team. You can't be one sided or limited. The Heat weren't versatile enough and Lebron has a long way to go in order to add to his game. We'll see if he's able to take that next step. In the meantime we have to keep improving the D and diversifying our offensive attack.