Details are still ketchy regarding the April 26 The Jesus and Mary Chain Coachella warm-up but it does look like Scarlett did not perform with the band on that date. She did, however, take the stage yesterday at Coachella Valley Music and Arts Festival in Indio, California to sing the backing vocals to Just Like Honey. Here are two reports that I came across:

Just when The Jesus And Mary Chain appeared to be winding things down, Johansson took the stage dressed in a colourful paisley mini-dress. Without saying a word, she joined Reid on the chorus of Just Like Honey.

“Wasn’t that nice?” Reid asked after they finished their duet and Johansson left the stage.
- From NME

Let’s get this out of the way: As widely predicted, Scarlett Johansson joined the Jesus & Mary Chain onstage Friday night at the Coachella Music and Arts Festival. But if that’s what you were waiting around for, you were probably disappointed.

The young actress came out wearing a paisley minidress and a straw hat toward the end of the legendary shoegazer band’s set, its first in eight years, to stand at center stage beside Jim Reid. Her job? To repeatedly sing the title line in Just Like Honey, a song recorded when she was an infant. Johansson looked a bit nervous and left the stage immediately upon finishing the song; she said nothing to the crowd. As to her vocal abilities, she didn’t do anything that anyone else couldn’t do. She may be a capable singer, but this performance didn’t prove anything.
- From A+E Interactive

I’ve also added 19 pictures of Scarlett’s performance to the gallery.