Changing bloginfo description from a plugin

You’re lookign for the bloginfo filter.

<?php
add_filter( 'bloginfo', 'wpse33522_change_bloginfo', 10, 2 );
function wpse33522_change_bloginfo( $text, $show )
{
    if ('description' == $show) {
        $text="Some New Description";
    }
    return $text;
}