Twitter has acquired social media startup Posterous – a micro-blogging site similar to Tumblr, designed particularly with mobile in mind, on which users can post text, images, and video to their Spaces.

The financial terms of the deal havent been disclosed, and both companies have stressed that Posterous Spaces will stay up and running, with “ample notice if we make any changes”, according to Twitters official blog post on the acquisition. But, with the same post promising instructions on how to back up content or move to another service, the future of Posterous doesnt look too bright.

So what does Twitter gain by bringing these developers on board?

“The Posterous team has built an innovative product that makes sharing across the web and mobile devices simple—a goal we share,”says the Twitter blog post. “We’re always looking for talented people who have the passion and personality to join Twitter. Acquisitions have given us people and technology that have enabled us to more quickly build a better Twitter for you.”

Could Twitter be looking at breaking that famous 140-character limit, and moving into offering longer, Tumblr-style microblogs? Watch this Space.