Most people think texting while driving should be banned. It's dangerous and stupid, yet many of us constantly do it. Today a new law goes into effect making the action illegal in the state of Tennessee, adding it to a growing list of states banning the action. The law does not delineate between texting, sending/reading email, or using the web browser. According to Tom Spangler, Chief Deputy with the Knox County Sheriff's Office, "the law has very good intentions but will hopefully get some additional clarification in the future. If you want to split hairs, any of that can fall under this new law."
In addition to sending text messages while driving the law also prohibits sending text while the vehicle is at a complete stop at a red light. Spangler said that if you absolutely must send a text message, pull off the road and park the vehicle. Law enforcement acknowledges that thew new law could be hard to enforce as it will be difficult to determine if a user is dialing a number or texting. (Update) As pointed out in the comments, a review of the actual law (pdf) states that this "shall only apply to a person driving a motor vehicle that is in motion at the time".
As of May 11, 2009 there were seven states including the District of Columbia that had passed laws making the action illegal with primary enforcement, five other states make the action illegal with secondary enforcement meaning the driver can only be pulled over for another violation. Several other states only prohibit texting and/or cell phone usage to minors, drivers with permits, and bus drivers.