This tweet from the Foo Fighters seems to indicate she may be---that's Joan's guitar in the middle.

Rumors have been swirling that Nirvana will do some kind of performance as they are inducted into the Rock and Roll Hall of Fame Thursday April 10th.  

Originally, we heard that R.E.M.'s Michael Stipe would be pulling vocal duties.  Who knows what Krist Novocelic and Dave Grohl have up their sleeves?!