Apparently, Google has developed software that uses a PC (or Mac presumably) with a Mic to identify background noises. This software creates a small digital fingerprint for each sound. Google can use this to target adverts based on (say) songs you listen to, or TV programs you watch.
Apparently, it can also fingerprint conversations between individuals, but Google claim that they cannot be reverse-engineered, so Google can't listen to them .
TBH, it would not be in Google's interest to spy on people in that way, but there may be people who would be happy to try..
http://www.theregister.co.uk/2006/09...ping_software/