How did They Do this? UINavigation with Toolbar on transparent

I'm just new to the ipad development world, and have come across the Instapaper app, and I just love the UI on this app.

Specifically, alt text http://sbox.smokingapples.com/wp-content/uploads/2010/06/instap开发者_运维技巧aper.jpg

Can you tell me what techniques would be used in creating a Navigation Controller with toolbar items, all on a transparent background?

How did they do it?

This seems like a common pattern in iPad apps, but I don't see any code examples in the apple developer site that gives hints on how this effect is achieved.

Thanks in advance for your answers.

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Provide your own implementation of -drawRect: in your navigation bar subclass, that doesn't draw a background image.