That rule was dropped a long time ago. AFAIK, the only remaining restriction is using Safari's rendering engine for browser apps.
Examples:
* Opera vs Safari.app
* Sparrow vs Mail.app
* Vimeo vs YouTube app
* Agenda vs Calendar.app
* Kik vs Messages.app
* Simplenote vs Notes.app
(I believe it came out that that "rule" was only used as an excuse for Apple to delay the Google Voice app. After they finally agreed to let Google Voice on the App Store a year later, I don't think I've ever seen anyone complain about it.)
It's actually not the rendering engine but interpretation of code downloaded from the netwOrk (i.e. JavaScript) which must use Apple's interpreter. That interpreter happens to be tied into their browser, so using it for other things is impractical.
Examples:
(I believe it came out that that "rule" was only used as an excuse for Apple to delay the Google Voice app. After they finally agreed to let Google Voice on the App Store a year later, I don't think I've ever seen anyone complain about it.)