I've been using Cocos2D to develop an iPhone application. It isn't a game, so Cocos2D will probably be all I need for my rookie attempt at iPhone development.
I would strongly suggest Objective-C. This isn't because I like Objective-C. (I don't) It is because Objective-C is structurally closer to Actionscript 3 than C++ is. C++ enables the ability to use OOP, but it isn't the most friendly envrionment for that kind of development. After learning some Objective-C, I would say that it has more in common with AS3 than C++ does. The biggest hurdle is the difference in the syntax. (Objective-C has some bizarre bracketed syntax)
I would definitely love to take Flixel for a spin on the iPhone. I've really enjoyed using Flixel in Flash, it makes getting most 2D games up-and-running much, much faster. (while still permitting acceptable flexibility in the design)