There's been a lot of discussion about the authority of Twitter users, and how users with many followers, or authority, or subject-matter expertise, might monetize their tweetstream via inserting paid advertisements. Here are the most important articles I've seen about this debate. I recommend reading them in the order below.

The New York Times has a piece that makes it sound cool and neat-o.

Paul Carr has a piece at TechCrunch that makes it sound like the end of civilization.

A venture capitalist investor in one of the services wrote a piece defending the idea.

Robert Scoble crunches some numbers and writes a good piece that digs deeper.

Finally, read this piece about the hypothetical SuperTweet with a "metadata payload."