Thanks to Hacker News I found a slideshare presentation from Amir Khella which shows "how to prototype like a pro".
He build a product called "Keynotopia" for Keynote and Powerpoint. It consists of templates that will help you building iPad, iPhone, Android, Windows Phone 7, Web, Facebook and UX prototypes.

Check out the presentation about Keynotopia:

And here is a video that shows how to prototype an iPhone app in 13 minutes:

I have bought it and will test it in the coming days. Maybe my dreams come true...