It might be the matter of authorization… I don't know, when it stopped working, but I am using role-based access control and last thing I've done with yiic was creating authorization hierarchy.
I commented out all checkAccess sections and nothing changes. Don't know what to do…
Did you verify that you can access your start page (default controller/action) as guest user? As soon as this is possible, yiic shell works for me.
Another idea might be, to activate logging for your application. yiic shell includes your index.php and starts the application as normal, but captures any output and discards it. So your app seems to exit somewhere in this process. Maybe you can see from the logs, what's going on.
I was redirecting from the home page to the login page, since in my application all users need to be logged in to access anything and everything on the site.
Therefore, i could not use the "yiic shell" tool.
Now whenever i need to use this tool i comment out the redirection, use the tool and then uncomment the redirection.