I was looking through games of the 2011 Tal Memorial and I found this particular ending shocking and disappointing. 

Unless I have missed something entirely, I believe that Carlsen completely blundred his lead. Then, soon after, Nakamura resigns (in what I believe to be a drawn position). I have not thoroughly looked this through, but I still believe my conclusion to be correct. If anyone could provide some insight as to what happened here, please do!

the black bishop defends the kingside pawns while the black king walks over to white's queenside pawns. white's bishop cant defend both sides of the board

I believe that you are incorrect in this. I believe this is drawn. But, even if you are correct, do you have any insight as to why Carlsen blundered like this? His blunder either prolonged the game or drew the game.

 He is correct it is losing for white. Whits bishop cannot defend the a pawn from the black king, and it is also incapable of stopping blacks king side pawns. The king cannot be in both places at once.

Can you point out the Carlsen blunder? I skipped through the game pretty quick but I didnt see anything.

i put the position in fritz 12 and clicked go until white resigned

Its winning for black. White is basically in zugzwang. The winning idea is: Bh5!!, then he moves his king along the light squares over to the kingside, and wins the pawns and gets a passed pawn. The h pawn is invulnerable, if ever Kh2 trying to win the h3 pawn, f2!, and white cant get back to stop the pawn from queening because the h3 pawn blocks g2 and and the f2 pawn blocks g1. And of course, the king cant move away from the kingside.

So white has to shuffle the bishop back and forth along the h2-b8 diagonal or the king has to shuffle back and forth between g1 and f2, and watch as black gets another passed pawn on the queenside and wins, and I'm sure naka didn't want to see that played out for him.