Late to the party here, but Guitar Hero is not done.
The only thing that's over is the annual market saturation -- if similar games like Rock Band stay popular, then you can bet Activision will go back to releasing Guitar Hero, albeit on a more limited basis and perhaps from a different team. It's all about the money, and the next Guitar Hero will be a bigger deal if the series is "resurrected" as opposed to just being another monthly episode.