If either you or your company use Twitter then I recommend you check out TweetEffect. This allows you to not only have general stats as to your opt in and opt out numbers, but it also allows you to see which 'tweets' won you followers and which lost you followers.
Of course - this is just the beginning. Just like examining your link stats on your email campaigns, you can use this information to identify what type of tweets your followers like and which ones they don't like, which will assist you in not only reducing your unsubscribes (as such) but also hopefully increase your number of followers.
Currently Twitter is still a 'new' toy as such and everyone loves trying it out, but as with most marketing mediums, the rule of quality over quantity still rules. So, think twice before tweeting - ask yourself are you adding value to your followers? Because if you're not -then there's plenty of alternatives for your followers to be following.