Expense and time tracking web application Harvest will announced tomorrow that it has added Twitter support. The new service will let users update tracking entries via Twitter from their mobile phones. The web page for the upcoming feature sports a "Powered by Twitter" logo and made us wonder: could this be a glimpse into a future business model for Twitter?

The Harvest-Twitter mashup works via a robot that users send direct messages containing new entries for Harvest. Once users link their Twitter account to their Harvest account, they can direct message commands to a Harvest Twitter client, which right now supports the adding of expenses (i.e., for adding a business lunch from the road) and the stopping of the software's tracking timer (i.e., if you just left the office and forgot to punch out).

It doesn't look like Harvest on Twitter supports starting the timer, though that would be nice for anyone who does work away from a desk that needs to be timed.

When Danny Wen of Harvest emailed us to let us know about the upcoming Twitter integration, he told us that the company looks at "the integration as a start of bringing more business use cases to Twitter." We agree, and further it could signal a potential business model for Twitter. While currently everything on Twitter is free -- from setting up an account, to running a bot, to using the API -- it seems plausible that Twitter could start charging for business uses.

Harvest is essentially using Twitter to power a quick mobile interface with their application and that's something that perhaps Twitter should start charging for.