I saw that there is now a system wide 'Tweet' function from the contextual menu in many places/apps in OS X. You can find it with a right click and highlight of any text.
Simply click and drag almost any text in Safari or Chrome, right click and select 'Tweet' from the menu. It tosses the text into the new tweet window in Twitter for Mac. I found it also works in apps like iChat, Mail.app, Ecoute, Reeder and iCal. These are just a few. Some things that it doesn't play nice with: the address bar in your web browser, text embedded in Flash, Firefox, just to name a few.
Troubleshooting information and the original post can be found here.
[crarko adds: I tested this, and it works as described. It shows up as a Service for me in those applications which support it. I checked the info.plist for the Twitter application (downloaded from the Mac App Store and it does indicate a 'tweetservice' so this is indeed provided by the Twitter application.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110125080925846