I have written a tab based iPad application which has done well. I never intended for it to be an iPhone application because the content it displays really doesnt lend itsel
I recently found this blog post on the topic. You might find it helpful:
http://www.raywenderlich.com/1111/how-to-port-an-iphone-application-to-the-ipad
Plus there's some discussion of creating a universal app here:
iPhone & iPad versions of same app?
iPad Programming Guide: Creating a Universal Application