http://drupal.org/node/606332

views-views-json-style-simple.tpl.php

<?php

// see http://drupal.org/node/606332

$jsonp_prefix = $options['jsonp_prefix'];
if ($view->override_path) {
  // We're inside a live preview where the JSON is pretty-printed.
  $json = _views_json_encode_formatted($rows);
  if ($jsonp_prefix)
    $json = "\$_GET['$jsonp_prefix']($json)";
  print "<code>$json</code>";
} else {
  $json = json_encode($rows);
  if ($jsonp_prefix && !preg_match('/\W/', $_GET[$jsonp_prefix])) {
    $json = $_GET[$jsonp_prefix] ."($json)";
  }
  if ($options['using_views_api_mode']) {
    // We're in Views API mode.
    print $json;
  } else {
    // We want to send the JSON as a server response so switch the content
    // type and stop further processing of the page.
    $content_type = ($options['content_type'] == 'default') ? 'application/json' : $options['content_type'];
    // Drupal 7: change to
    //drupal_add_http_header("Content-Type", "$content_type; charset=utf-8"); 
    drupal_set_header("Content-Type: $content_type; charset=utf-8");
    print $json;
    exit;
  }
}