I can see a usage based charge system where a certain amount is free, and then you are charged based on usage after that. Imagine a Heroku model for Twitter. Many users can get by with the free model, while businesses will likely be the ones going over the free limit and into the pay-per-tweet area.

Imagine you get 3,000 posts per month free. That's an average of 4 posts per hour every 24 hour period (6-8 more likely, because sleep). Most users won't come close to that, but businesses likely will depending on their usage and nature. You could even go the wireless phone route where you buy a subscription of messages in blocks, like 6,000 or 10,000 etc. This allows for monetization without punishing those who are experimenting or casually using the service.