If you get this error after doing an upgrade to your WordPress blog here is what you need to do. The error is “Fatal error: Call to a member function add_rewrite_tag() on a non-object in” what you need to do is simple enough.
Start by disabling your plugin folder by renaming it old-plugins, create a new plug in folder. Then go back into your WordPress install and it should come back to life now. click on plugins and you will see a bunch of errors as it disables all your plug ins because it cannot find the files for them.
Now go back and delete that folder you created and rename back the old-plugins to plugins. Now go back to your WordPress install and reactivate the plugins one by one testing your install for errors after each activation. As soon as the error reappears you have found the plug in that is at fault.
In my case it was the wp-ecommerce plugin causing the issue due to using a custom permalink structure. I changed it back to the default permalink structure and it works fine now. Since I was not really using the custom structure on that site I will get back to solving it completely later
If you have questions or need help feel free to leave a comment her thanks.