The official Twitter client for Mac OS X, who was born Tweetie, has a hidden options pane. And it is this pane that has the option to show full names of the user in your Twitter feed, and not just the user names. It is possible to change to full name-view by entering this line in Terminal: defaults write com.twitter.twitter-mac ShowFullNames 1. To disable, use 0 instead of 1. But to get this option into a menu, enter this line in terminal instead: defaults write com.twitter.twitter-mac DebugMode -bool true.
And hey presto! Restart Twitter, and you will have a new menu under preferences.
This was originally meant as a secret thank you to those of us that had bought Tweetie before it transformed into the free Twitter client it is today. But why it hides such a vital option as view full name is beyond me.