Yes there is an API for that. see this.
I recommend using the JSON API as it is very easy to parse with the objective-c framework. Look at the fourth section on the page I linked to, under 'Friends and Followers' and you'll see there are requests you can make to access followers and following.
For example, this is how you get someone's followers:
http://api.twitter.com/1/followers/ids.json?id=myuser
This returns a JSON array of user IDs which you can use in conjunction with other APIs to get their user info.
Hope this helps you getting started
