I've installed the plugin, but unfortunately it works incorrect with mobile browsers. I've got the newiest versions of all browsers on my Android device, but this plugin makes popup in almost all of them (e.g. Opera Mobile). So I had to turn this plugin off...
But the idea of plugin is great! I hope that author will fix this little problem.
http://wordpress.org/extend/plugins/push-up-the-web-for-wordpress/
