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The performance of John Williams' "Air and Simple Gifts" at President Obama's inauguration was actually a recording. Violinist Itzhak Perlman, cellist Yo-Yo Ma, clarinetist Anthony McGill and pianist Gabriela Montero played along to the recording, inaudible to all but the few spectators seated near them, for the benefit of television cameras.

It was too cold for the musicians to give a reliably good live performance, The New York Times is told:

http://www.nytimes.com/2009/01/23/arts/music/23band.html?_r=1&hp

Appearing to give live performances of pre-recorded music is commonplace at big public events that are televised, assorted experts explain.

And no, Ma did not play the carbon-fiber cello, widely touted before the event.

Other than that . . .