Ok, I would create a xml file with this code...
<modification>
<id><![CDATA[JSON infor for APP]]></id>
<version><![CDATA[0.8]]></version>
<vqmver><![CDATA[2.3]]></vqmver>
<author><![CDATA[Jeremy Fisk (jeremyfisk@clear.net.nz)]]></author>
<file name="catalog/controller/product/category.php">
<operation>
<search position="replace" error="log"><![CDATA[$this->response->setOutput($this->render());]]></search>
<add><![CDATA[if(isset($this->request->get['json'])) {
echo json_encode($this->data['products']);
die;
} else $this->response->setOutput($this->render()); ]]></add>
</operation>
</file>
and then save it in the vqmod/xml folder... this means that if you navigate to http://example.com/index?route=product/category&path=26 it will show the page like normal, however if you add the json atribute to the url you will get the json data... the url for this would be http://example.com/index?route=product/category&path=26&json
This same code could be modified to add json data for the cart, for checkout, for product pages etc etc...
I trust this is off assistance to you and i look forward to hearing how you get on...
Regards
Jeremy