Alexander Fleming gave the following warning, way back in his 1945 Nobel speech:
It is not difficult to make microbes resistant to
penicillin in the laboratory by exposing them to
concentrations not sufficient to kill them, and the
same thing has occasionally happened in the body.
BBC did a very good radio segment about the penicillin discovery in their "50 Things That Made the Modern Economy" radio series: http://www.bbc.co.uk/programmes/p04pfn2z