Last week, I discovered on my www.twitter.com a new button "RT".
I've explained it in this post why I think it's nice even if .. there's a limitation :you cannot comment anymore your retweet (add a personnal touch).
In this post, @ev explains the reasons of this implementation, and the reasons are very good, and especially it improve the overall system consistency.
With an embedded RT function, the whole system will be consistent, and it will be possible and easy to see who is retweeted, who are the sources.
Today, it's not really possible, because every RT alters the initial message.
@ev also explains that it's a first version, and that improvements will come later.
I let you check read the post for more details.