EA has admitted that recently, a lot of people were banned in Battlefield 3 on PC in error. So if you were one of those unlucky people, listen up.
As per the message on EA's Help site, "We are investigating an issue of Punkbuster bans that were incorrectly applied to some of our players. Please understand that our game advisers are not able to access or overturn Punkbuster bans, but we're working with our partners at Even Balance to get this resolved as quickly as possible."
If you've been banned, go to this site and appeal it.