How private are my conversations?
The answer is, "that depends." You may have a reasonable expectation of privacy in your conversations. A reasonable expectation of privacy may exist in a person's home, but statements made in the presence of "outsiders" are not protected since the statements are in "plain view" with no intention to keep the matter private. Further, conversations held out in the open that are not protected from being heard would not be considered private.
With most telephone conversations, there is a reasonable expectation of privacy (although a possible exception to this is when a public telephone is used). Cellphones have made it easier for strangers to eaves drop in on others' conversations and a person has no legal rights to privacy if they are in public and openly speaking on the phone.
Secretly recorded conversations require further analysis. Federal law prohibits the intentional recording of oral communications under most circumstances. One notable exception to the federal law is when the person making the recording is a party to the conversation, or has consented to the recording (so long as the recording is not for the purpose of committing any criminal or tortuous act). Thus under federal law it may be permissible for a person to record his/her conversations with others and then use the recording as evidence.
Some states have laws that differ from the federal approach. For example, in the State of California it is a crime to secretly record a conversation (unless all parties to the conversation consent to its recording). Law enforcement officers are entitled to record conversations under both federal and state law when certain conditions are met.
Police Recorded Conversations
The law on this matter varies per state, but in some states, such as Illinois, although it is illegal for citizens to record a police officer with an audio or video device in public, the law allows police to record anyone, without a warrant, during drug investigations.
Under federal and state laws, it is generally required for police to obtain a wiretap warrant in order to record telephone or private conversations. This warrant must be obtained with a judge's approval and only after cause is shown. Some state laws, however, have some loopholes regarding text messages; it is implied that wiretapping laws only require a warrant to listen in on a real-time conversation, not use a recorded message to implicate a suspect to a crime, or further an investigation in another manner. In some cases, police have used one person's text messaging to entice a suspect to a location or to gain information to further a case. The law interprets text messages as having been sent out into the world - making them no longer a person's property that would otherwise be protected under the Fourth Amendment from being seized by the police.
If you are concerned about your privacy from others or from authorities, it is in your best interest to have important conversations in the privacy of your home and in person. If you are facing a possible criminal charge and you suspect the police may have reason to obtain a warrant, there is little you can do to prevent this action as law enforcement has legal authority to wiretap your conversations or seize your phone.