"Indeed, securing the proper approvals was no small feat. Not only did Cobain?s widow Courtney Love have to sign off on behalf of the estate, so did former drummer Dave Grohl and Primary Wave Publishing, which administers the Nirvana catalog. 'There?s a lot of moving parts to something like this,' Riley explains. 'But it was about timing, not an issue of resistance. We almost did it for World Tour, but we couldn?t get it together. Then after three years of working with the different parties, it was like the perfect storm.'?
Wow, she is completely nuts. That article goes on to say that she even helped pick out his clothes and hair style in the game.
http://www.rollingstone.com/rockdaily/index.php/2009/08/27/kurt-cobain-joins-guitar-hero-5-how-nirvana-came-to-the-game/