Selected Quotations From The iPhone HIG
Providing a custom navigational control encourages users to ignore the built-in button bar while they are in your application, decreasing the possibility that they will mistakenly tap the Safari on iPhone back button and strengthening the feeling that they are using a standalone application.
I get that you don’t have the API ready, or don’t want to share it, or whatever you’re reasoning is. I don’t really care anymore. But please stop insulting my intelligence. Stop telling me how I should be happy to run my code in a browser and then turning around and telling me that it’d really be a good idea if the user didn’t notice how they were in a browser.