In this WPShout entry Angela Giese provides the code to use and retrieve the number of your Twitter followers without being hit by the Twitter API restrictions, which popular bloggers can suffer from. This work around is mainly aimed at popular sites not using the Twitter widget.
Once you have added the code to your functions.php you can easily implement the number of Twitter followers anywhere in your theme or design.
Read the tutorial, completely with copiable code, at WPShout.