WordPress MU

Is it possible to somehow change the default setting of this field (under edit blog) to the blog keywords or description field? Instead of "just another domain.com blog"?

Just put an update_option in your "wp-includes/wpmu-functions.php" inside the "install_blog" function:

update_option('upload_path', "wp-content/blogs.dir/" . $blog_id . "/files");
update_option('blogname', $blog_title);

//Begin another blog description
update_option('blogdescription', 'Another blog description here');
//End another blog description

wpdb->query("UPDATE $wpdb->options SET option_value = '' WHERE option_name = 'admin_email'");

Or go to wp-admin/includes/schema.php and edit this line:

add_option('blogdescription', sprintf(__('Just another %s weblog'), $current_site->site_name ) );