With this Plugin you can set several descriptions for your blog. They change randomized without reloading the page.
Extract the zip file and just drop the contents in the wp-content/plugins/ directory of your WordPress installation and then activate the Plugin from Plugins page.
Be sure, that in your header.php the code is as follows:
< div class="description">< ?php bloginfo('description'); ? >
The important thing is the class "description"!!