It's just Paul McCartney playing with the members of Nirvana, not quite how the media are portraying it. Paul played with Dave Grohl at Anfield in 2008, The Grammy's in 2009 and The Grammy's 2012. Grohl also sang Band on the Run at a tribute to Paul at the White House in 2010, Maybe I'm Amazed at The Kennedy Centre Honours McCartney tribute in 2010 and more recently the Foo Fighters performed Jet at the MusiCares Person of the Year 2012. They are also rumoured to have got together in the studio to make some "garage music".
They are good friends, just a case of the media blowing this out of proportion.
As others have said, I hope Paul is in good voice. Things can't get much worse for him than the two massive screw ups this summer.
I'd like him to do something a bit more unusual but I know it'll probably strictly be hits at a show like this.