Padpaw API

From their site: Padpaw helps groups communicate and organize. Built around the existing capabilities of your mobile phone, it delivers timely information and messages to group members. Because it's mobile, Padpaw can alert members to changes in events and plans instantly through multiple channels; SMS, email, and soon IM. Padpaw also provides a message board that lets the organizer and members communicate with each other. The amount of information a member gets, and how they get it, is under their control.