I'll be building it as a webapp using the usual JS/CSS/HTML, but will be porting it to mobile using PhoneGap.
In short, the application is a public, location-dependent, ephemeral, bulletin board. A user can anonymously post a text snippet, photo, or 10s video. When a post occurs, all users will be able to view the current post until a new one is posted. Old posts are deleted as soon as a new post occurs. Further, users can only see posts made within a particular radius around them.
Progress: Since starting, I've implemented live snippet posting. I will be aiming to port to PhoneGap before implementing images and video.
Current Build: https://jamapp.herokuapp.com/