It's on multiple carriers globally and being outsold about 2 to 1, that despite the fact that IPhone has a much larger percentage of existing upgrades in its sales numbers. If you look at the trends, it's pretty clear that BB and Symbian users are choosing Android by a fairly wide margin. That IPhone share has been relatively flat even while expanding carriers suggests saturation, while continuing to capture a fairly constant portion of new smartphone sales.
And the multiple carrier/multiple devices thing is really about trade-offs. The Iphone brand clearly benefits from the exclusivity and simplicity (quality control!) of one device on fewer carriers. IOS on multiple devices would significantly cannibalize IPhone sales, and IPhones running Android or another OS would significantly cannibalize IOS share.
Android and IOS are really both very good OS's and it's splitting hairs to argue one is better than the other. In a free for all they probably would split the majority of the market, but that would necessarily bring the IPhone back closer to the pack in terms of the top handset.
And if Nokia partners with WinMo this probably becomes a 3-horse race.