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."Nextel says that they didn't participate in the eavesdropping on a couple of mobsters who were allegedly listened in on using this technique--both using Nextel cell phones.
The same story reports that a 2003 lawsuit revealed similar monitoring of conversations occurring in cars featuring OnStar.
UPDATE (December 5, 2006): Bruce Schneier has commented on this story, and his readers have some interesting comments.