Dave Winer posted today about how much the #dickbar sucks, and more importantly, how rudderless Twitter seems to be in finding a solid business model.
Well I've got one, and it's quite simple. Allow accounts to charge to be followed. A one time fee, or a subscription. Twitter takes a cut, say 30% like the Apple app store.
This is ideal for a platform as it creates new business opportunities for Twitter users. eBay created all kinds of small businesses, as has Amazon, Etsy, Google, etc.
The most obvious uses at first would be things like stock picks or selling subscriptions to digital goods.
It could also be used by sites as a very lightweight payment mechanism. Follow this account (Twitter handles payment) and you then get access to an app or website which is using the Twitter API.
It would also be a way for people to support things they care about in a public way. I can follow the paid account for a band I like, which gives me special access, exclusive mp3s maybe, but it also puts an icon on my profile showing that I support the band. There's a lot of social/karma/reputation value in that. Similarly, it could be used by non-profits as a way to raise money, maybe Twitter would even waive the fees.
If done correctly, it would be the simple micropayments and subscription mechanism people have dreamed about for years but no one has ever been able to pull off. Twitter is in the perfect position to do it.
UPDATE: A few more thoughts based on great feedback:
I'm not too concerned about retweets. New-style retweets could just be disabled for accounts that require payment, just like protected Twitter accounts already have retweets disabled. Old style RTs could be identified a good portion of the time via algorithms, not unlike how Twitter has to deal with spam. If YouTube can identify copyrighted content in videos, Twitter can certainly identify it in 140 chars. Google has to deal with click fraud, every business model like this needs anti-fraud measures.
Some have brought up how easy it would be to repost content on external sites, but people already do that for every type of digital content available, and it's not the end of the world, so I don't think it matters. iTunes sells plenty of music and you can still download all of it on the Pirate Bay.
This could be as simple as clicking the "follow" button and confirming the charge. People would keep their credit card information on file (like they do with iTunes), so Twitter would have to tighten up security and make sure people trusted them with that info. They are already adding HTTPS support, so they are well on their way.