At the All Things Digital Conference, otherwise known as D7, Twitter Co-founders Evan Williams and Biz Stone talked about possibilities for making money in the future, but once again, did not want to give too many details away.

Williams said the company is working on a form for people to fill out to give them money, so evidently something is in motion. TheStreet.com reports:

Williams and Twitter co-founder Biz Stone mentioned possible revenue-generators, including a service that would authenticate the source of information. For example, Dunkin' Donuts could pay to make sure that impostors don't send messages under its name.

Still, after nearly one hour of questions from journalists Walt Mossberg and Kara Swisher and from the audience, the co-founders gave no clear picture of Twitter's business model. Stone demurred when asked what would be the company's key revenue driver in two years.

Williams added that he isn't entirely opposed to banner ads, but said it's probably the least interesting thing the company could do. ZDNet has some footage from the All Things Digital Conference here.