I created a website that announced the launch of a small mobile app for both iOS and Android platforms. I was told by Apple that my app would not be approved as long as I had links to competitors (i.e. Android) on the web site.
How’s that possible? There’re a lot of (thousands?) multi-platform apps for iOS, Android, Blackberry etc. For instance, Angry Birds has versions for “iPhone/iPad/OS X,” “Nokia Devices,” and “Android (Amazon/Google Play)” on their website: http://shop.angrybirds.com/us/games/mobile-games.html
Needless to say, the full list of platforms Angry Birds runs on is (according to Wikipedia) much bigger.