Cecil’s column http://www.straightdope.com/columns/000630.html (“Does nonoxynol-9 protect against HIV?”) hit the proverbial nail on the head.
Check out http://www.cdc.gov/hiv/pubs/mmwr/mmwr11aug00.htm for the CDC’s latest position. It’s pretty scary stuff.
Oh, and if one has been in positions where nonoxynol-9 was thought to be protecting, better see the local free clinic.