When you post from an application like CoTweet, TweetDeck, or Twitterfon, the tweet posts a statement containing the date of publication of the tweet and at what source you have posted the message from. So in the above Coke example you will see that they posted this tweet from CoTweet. The actual text "CoTweet" links back to the CoTweet.com site. So why not do what all of these other applications do and create the tweet from your own domain?
If Coke was forward thinking they would have created a Twitter application on their own domain and thus have the capability to brand this "from" link. This would totally brand the entire tweet. Why allow these third party apps to get all the glory?
Now your are asking do I need to put in the time and resources to create my own Twitter application? Well not exactly. A French programmer developed a PHP script to allow you to set up a brandable Twitter "from" link. The translated article can be found here. Now the script requires PHP5 and an Twitter oAuth account. This script was originally designed to exploit the fact that these "from" links were DoFollow links. Not sure if this script was the reason why Twitter recently closed that loop hole and started to noFollow these "from" links (first reported here).
I recently tested this script and here is a screenshot of my Tweet.