As I've said before, it's a little problematic to compare these new mobile platforms at this point, because Android has no shipping devices, and the iPhone native SDK has not yet been released. However, it appears that several hardware vendors are working on shipping real Android devices in the second half of this year (and some hacks have already shown that a stripped-down Android can run even on older hardware). My gut feeling is that because Google is creating a more open atmosphere around the Android platform, we're likely to see a wider variety of Android devices sooner, possibly leading to a huge volume of new Android users. By comparison, Apple is clearly keeping a tight grip on the native iPhone platform, and it's unlikely that they would endanger the iPhone user experience by allowing third parties to ship iPhone devices. So while the iPhone is wildly popular today, it may take many years before the number of iPhone users matches the number of e.g. Series60 users.
Then again, if every iPod Nano user upgrades to an iPhone...