No, they are two different formats. The main difference is that I2S contains both data and the clock (separate wires), while with S/Pdif you have to extract clock from the data stream (one wire). Yes, you can have multiple devices on I2S bus, but the main reason for it in audio is to have better (less jittery) control of the clock.
I thought I2S was used internally in a piece of audio equipment, and S/PDIF was used to transmit it to another piece of audio equipment. Seems like Sony and philips would have used a better format than s/PDIF.
