I would would like to request the ability to specify the output. Specifically JSON as the result. It is much easier for many platforms to work with JSON than XML. For instance PHP 5.2+ has two built in functions json_encode() and json_decode(). Javascript developers can just eval the JSON and get a javascript object.
More resources: http://www.json.org/
Simple fix for the API output:
if(strncasecmp($_POST['output'], 'json') === 0) {
$output = json_encode($response);
}
// Other formats...
else {
$output = // XML output
}