In a new post to his blog Giulio Pons talks about how to send push notifications to an iPhone using the pushme.to application installed on the user's phone.

Push service is a technology that allows you to send alerts/notifications to a mobile device. Blackberry has its own push service, iPhone has its own, and also Android devices has their own push services. [...] I want to send push to my iPhone, but I’m not able to develop an iPhone application that receive push alerts, and I don’t want to send only email alerts.

You set up the application on your phone and make a widget on their site for your application to use. Included in the post is also the PHP you can use to connect to the pushme.to service (using a curl POST request to send form encoded info to their servers).