It's official now!!!
And that is not the only date for the U2 tour with Muse in the States:
"Autumn tour news
We are pleased to confirm that Muse will be touring in the UK, Europe and North America this autumn. Full details on the UK and European dates will be announced soon, but for now we can confirm that the the band will be special guests to U2 at a number of their North American shows this September including New York on September 24th. The remaining U2 dates will be announced in the coming weeks."