From the Chicago Federation of Musicians:
Maestro Muti will join the musicians of the Chicago Symphony Orchestra, including a brass section that will perform, and others in solidarity at the Picket Line for a press conference — Tuesday, 10am at Symphony Center, Outside, at 220 South Michigan Avenue.
It is, we think, unprecedented for a US music director to go on strike with the musicians.
UPDATE: We understand that Mstislav Rostropovich went on the picket line with the National Symphony Orchestra in 1993 after some musicians were accused of violating the police perimeter. He stayed for half an hour and the strike continued for six weeks.
UPDATE2: Muti clarifies