Got is interesting piece of news through POLITECH (Politech is the oldest Internet resource devoted to politics and technology):

FBI taps cell phone mic as eavesdropping tool
December 1, 2006, 2:20 PM PST

The FBI appears to have begun using a novel form of electronic
surveillance in criminal investigations: remotely activating a mobile
phone's microphone and using it to eavesdrop on nearby conversations.

The technique is called a "roving bug," and was approved by top U.S.
Department of Justice officials for use against members of a New York
organized crime family who were wary of conventional surveillance
techniques such as tailing a suspect or wiretapping him.

Nextel cell phones owned by two alleged mobsters, John Ardito and his
attorney Peter Peluso, were used by the FBI to listen in on nearby
conversations. The FBI views Ardito as one of the most powerful men in
the Genovese family, a major part of the national Mafia.

...

Nextel and Samsung handsets and the Motorola Razr are especially vulnerable to software downloads that activate their microphones, said James Atkinson, a counter-surveillance consultant who has worked closely with government agencies. "They can be remotely accessed and made to transmit room audio all the time," he said. "You can do that without having physical access to the phone."

Now, call me stupid, but I do not exactly know how this would be technically possible on for example on Nokia phones (although Nokia is mentioned in the article quoted), as the phone UI would reveal an active connection to the user. And if a separate application would be used, the user needs to install it first (or did they also replace the phone with a new, bugged phone at some point). Anyone care to enlighten me?