[Edited after the game to add video of Jeremy Roenick and Mike Milbury discussing the hit.]
Here's the video:
Thoughts on the hit after the jump, as well as the video of the post-game discussion
First, a few primers. Letang is the last person to play the puck and he's also attempting to continue to make a play on said puck, so he is completely eligible to be hit. Nystrom is under no obligation to play the puck, and he is free to take only the body since Letang is the puck carrier. This is not interference.
Now, Nystrom received a penalty for roughing on the play. That is also not applicable for the play itself. Roughing is, and I quote, "Roughing is a punching motion with the hand or fist, with or without the glove on the hand, normally directed at the head or face of an opponent." That obviously didn't happen on this play. I am nearly positive Nystrom actually received nothing from the hit and was the only player picked for a penalty out of the post-whistle scrum.
The real question will be whether or not Nystrom violated the illegal check to the head rule, which you can find here if you are so inclined. While Mike Milbury may disagree, I think the fact that Letang lowers his head and lunges in the moments before impact will save Nystrom here. Brendan Shanahan has been very clear that if the player who is hit makes a move immediately before contact that puts his head at the level of impact, the fault is not on the checker.
Basically, it's playing a semantics game of how soon "immediately" is. I think it's a clean hit, but I'm biased.
While Milbury thinks it's a suspendable hit, Jeremy Roenick was absolutely certain it was not. They two had a very heated discussion on the post game show, which you can see after the jump.