I wish there was a way to end this meme, so people didn't keep saying the same dumb thing every time: Changes to Plan 9 are continually integrated, with a new ISO being generated every day. Inferno lives as a Mercurial repository these days, and it too gets updated frequently. In Plan 9, for instance, they recently added a better bootloader and support for timed semaphore acquires (and after that patch, you can now build Go for Plan 9). We even get ports to new systems from time to time, like the beagleboard or the gumstix.
At the speed Linux changes, a 5-year-old Inferno would not even build. The lates available tarball is from 2010, but the reason a tarball is even provided is because it speeds up a pull from Mercurial. If you are interested in the current goings on, I've summarized some of them: http://debu.gs/entries/plan-9-and-inferno