Back in February Carter Buwell confirmed via twitter and his official site that Rob was going to play on the piano 'Renesmee's Lullaby' in Breaking Dawn - Part 2.
Now, Jack_Morrissey tweeted a few more details about the scene :)
In March, during his interview with MTV, Rob talked about playing the song on the piano
We'll have to wait until November 2012 to see that. It'll be worth it! :D
The picture used in this post is from Twilight, not Breaking Dawn - Part 2