Kutaragi also talked to Nikkei about various aesthetics of the Cell chip. He commented that the microprocessor comes with eight Synergistic Processor Elements (SPEs), since the number eight is an "exponential aesthetic." He also added that the Cell uses only seven of them since that's "the ultimate aesthetic" in an engineering sense; at E3, Sony revealed that ignoring one SPE as a redundancy will improve the chip’s yield and allow production costs to drop dramatically.
http://www.gamespot.com/news/2005/06/17/news_6127714.html