The original Sinclair ZX Spectrum manual featured a listing to play a few bars of Mahler's First, with the reader asked to play the whole piece as an exercise. Here's Matt Westcott completing that exercise with the help of a few friends and a table covered in Spectrums. In the process of making this possible it is also believed that the record was broken for the number of networked Spectrums in one place.

I can't get youtube (at work)

Is this the event in Oxford, with a bunch of speccy 48s hooked up with one of these? http://spectrum.alioth.net/doc/index.php/Spectranet

(I think there would have been a Raspberry PI in the mix somewhere as a network metronome)

Indeed it is, and using the Spectranet board. The Pi simply provided synchronisation, each Spectrum had the whole piece in memory as BASIC code.

We had a bit of a discussion about how it might have been done back in the day using 1 bit input ports and a Spectrum doling out sync pulses.

Edit: Though the Spectranet's auto-loading of the code made things far easier than it would have been with loading 12 Spectrums from tape!

Fabulous, I've been waiting for the video since I saw the photos on facebook :)

Actual title:

    Mahler's first symphony, as played
    by 12 networked Sinclair ZX Spectrums

The first 7 minutes are explanations of the challenge and how it came to be. Actual "performance" starts at the 7 minute mark.