In Pennsylvania, if you tape a person to person conversation without the other person knowing, it's a crime.
"It is a felony of the third degree to intentionally intercept, endeavor to intercept, or get any other person to intercept any wire, electronic, or oral communication without the consent of all the parties. 18 Pa. Cons. Stat. ยง 5703(1)."
From this site:
http://www.rcfp.org/taping/
The laws vary DRAMATICALLY state to state. Know what the laws are in your state before you do something like this without telling the other person in the conversation.