Here's a news article on it:
http://news.com.com/FBI+taps+cell+phone+mic+as+eavesdropping+tool/2100-1029_3-6140191.html

Security-conscious corporate executives routinely remove the batteries from their cell phones, according to James Atkinson, a counter-surveillance consultant who has worked closely with government agencies.

It's not just the FBI either, apparently:
http://www.thenewstribune.com/front/topphoto/story/91460.html