mmuk_replace_howdy_message

function mmuk_replace_howdy_message( $wp_admin_bar ) {
    $my_account = $wp_admin_bar -> get_node( 'my-account' );
    $newtitle = str_replace( 'Howdy,', 'Logged in as', $my_account->title );            
    $wp_admin_bar -> add_node( array(
        'id' => 'my-account',
        'title' => $newtitle,
    ) );
}
add_filter( 'admin_bar_menu', 'mmuk_replace_howdy_message',25 );