add_filter( 'admin_footer_text', 'theme_admin_footer' );
function theme_admin_footer()
{
   $theme_name = get_current_theme();
   echo '
   <span id="footer-thankyou">
      Using the ' . $theme_name . ' ' . the_theme_version( false ) . ' theme by <a href="http://www.merovingi.com/">dbm</a> &bull; <a href="http://codex.wordpress.org/Main_Page" target="_blank">Codex</a>
   </span>';
}
function the_theme_version( $echo = true )
{
   $themeversion = get_theme_data( get_stylesheet_uri() );
   if ( $echo ) echo $themeversion['Version'];
   else return $themeversion['Version'];
}