I haven't done anything for the iPhone, though I'll have to eventually (unforunately). I've heard XCode or ObjectiveC isn't the most fun to work with. I don't have a Mac, so I don't know much about it.
I have worked with Android though and have viewed my Flex apps on my Incredible. Haven't been impressed with how they function on the phone. First of all if I click on a button (or other object), I always have to tap off of it to get it to click. Though that is using the 3.5 SDK, not 4. The map was also hard to navigate. I know there is Air for Android, but I haven't looked at that yet, not sure if there are plans to do something similar for the iPhone. For the Android, i personally prefer making native Android apps using Eclipse.