>if you go to another country to have sex with a child prostitute, you can still be charged with a crime in the US, or so I was told on some message board

Now you've been told on this message board.

Mann Act http://en.wikipedia.org/wiki/Mann_Act

Protect Act of 2003 http://travel.state.gov/travel/cis_pa_tw/cis/cis_1467.html

http://en.wikipedia.org/wiki/PROTECT_Act_of_2003