Like twitter or facebook but open, decentralized, and unarchivable would be nice. Something along the lines of Off The Record (not google talk's) where users could disavow messages from their account manually or automatically after x number of days. It seems like twitter is useful for asynchronous communication with a very short shelf life, as opposed to IM (synchronous) or RSS (hopefully a longer shelf-life), which would lend itself to that sort of privacy model.

Disclaimer: I have no idea if this can be implemented.