the twitter example you gave is perhaps too simple, could the tweets not just be text written to a publicly writable file. the users could connect with 9p but as the user none son they will need no auth.
better examples of the everything is a file aproach are wikifs (a file server which prvides virtual files for the httpd server (or any normal 9p file client) to access and stores a database of wiki pages. http://plan9.bell-labs.com/magic/man2html/4/wikifs the cassic example is using a plan9 server as a gateway machine across a firewall. This machine is dual homed and all machines inside the firewall are isolated. when one of these machines wants to connect to somone outside the firewall they can just import the gateway's /net.alt over the top of their own. (by tradition the primary interface is mounted on /net and the seccondary at /net.alt) Now any DNS lookup and socket connection will be made using the gateway's internet facing NIC. This is all done using the 9p protocol, no clever IP routing etc. if the 9p connection to the hateway happens to come over an ssh session, ppp, or pigeon, it doesn't matter, you are sharing files, and these particular files give you access to that machines network interface. there is also a really neat trick you can use do a similar thing with a unix machine as the gateway - sshnet provides a /net like interface to plan9 but uses ssh's remote port forwarding to speak to unix: http://plan9.bell-labs.com/magic/man2html/1/ssh -Steve
