Why is this such a Big DealTM? @replies becoming a part of the API were a big deal because you could follow the conversation thread using them, RTs don't really have any other context and we're already doing pretty well reposting a tweet with RT <nickname> in front of it.
The only important ramification I can now see is that it will become easier to track what content is being spread from person-to-person without having to rely on searching for "RT" or "via"...