Do we still need to extend the quaint courtesy of letting people know where we found a link from another Twitter user? I think the retweet, aka RT (in both its informal and official forms), has run its course.
There are now over 200 million Twitter accounts (not all active), with nearly half a million being added daily. There are over a billion tweets a week. As tens of thousands of print publications have opened up shop online, and with the continued growth in things like personal blogs, Posterous, and Tumblr, there are now exponentially more sources for links than there were two years ago.
I think it's time to recognize that Twitter is the world's largest and most important source of community validated information. That information can stand on its own. It is not important to know who posted it last, only where it was published first.
I understand the appeal of the RT from an etiquette standpoint, and I do use them most of the time unless there are two or more original tweeters or the format of an RT eats too deeply into the space available for the actual information in the tweet.
I also realize the RT underlies Twitter popularity and "influence" measurements across the social media world, but does it really tell us much? We're still in a 140-character world. Do we need to trip over ourselves to add the RT information to a tweet that didn't originate with the person we are retweeting in the first place? There is so much information moving through Twitter, around 140 million tweets a day. Does the RT really carry that much useful information?