I am pretty excited about using Posterous in the classroom. I plan on using it as a Concept/Question board (a component of Open Court language arts). With parent permission, I will collect students' email addresses and add them as contributors. The site is password protected for student privacy, so only students and parents that are given the password will be able to access the site. Each student will be required to contribute at least one post and two comments on others' posts during the unit.
The current Concept/Question board is a boring bulletin board in the classroom. Students are not jumping to contribute to it. Using Posterous will motivate them to share many more things with the convenience of simply sending an email to email@example.com.
Posterous would be an excellent tool for a telecollaborative project. Managing teachers could set up a group page and add the students in the group as contributors. As contributors, the students could post their work progression/components of the project as documents, photos, helpful links, movie files, etc.
This site contains examples, guides for teachers and students, and tips and ideas. You may need to click on "Posterous" on the left hand side. They also offer this brief 3 page guide on how to use Posterous in the classroom. It was extremely helpful in setting up my Posterous. Remember, you will need a password to view the site. It is "sprinkles". Also, Richard Byrne's blog for teachers offers the idea of using Posterous for students to create online portfolios, with each entry dated.