The latest Twitter blog post, The Twitter Platform, discusses Twitter's long term strategy for their advertising business model. Buried within the middle of this post is a reference to paid tweets.
"...we will not allow any third party to inject paid tweets into a timeline on any service that leverages the Twitter API. We are updating our Terms of Service to articulate clearly what we mean by this statement..."
It looks like the end to third-party sponsored tweets. A wise man once said, "Never build a business model off another business' API.".