If London is any example, cameras don't actually work that well:

    a study of London's widespread use of CCTV cameras, found that for every
    1,000 cameras installed, only one crime has been solved. On top of that,
    when faced with a crime, the CCTV cameras are rarely that useful. The report
    found that CCTV cameras were used to catch just 8 out of 269 suspected
    robbers.

http://www.techdirt.com/articles/20090827/0410406023.shtml

This is without mentioning the privacy aspects; I find it rather worrying how people seem to accept constant surveillance so easily.