I had an old version of WordPress Mobile Edition installed along with the Carrington mobile theme, but I recently changed to a different plugin. Also after upgrading to WordPress 3.x a while back, I noticed the plugin had stopped working and was no longer visible on the plugins page.
This morning I started off by deleting the Carrington mobile theme from the Themes page in the WordPress admin console.
The plugin, however, requires some manual effort to fully remove it.
First you need to FTP or SSH into your web host, and go to the wp-content/plugins directory.
Delete the wp-mobile.php file.
After doing this you will have an error stating that the wp-mobile plugin was deactivated. This nag message will not go away. To remove the nag message and complete the removal of the plugin, you will need to log into the WordPress database (MySQL, Postgres, etc.) using myPhpAdmin or other database tool, and execute the following SQL command:
DELETE FROM wp_options WHERE option_name LIKE ‘wordpress_mobile_%’ OR option_name LIKE ‘cfmobi_%’;
Once the WordPress Mobile options have been removed, the nag message will no longer appear and you can get on with your day.