Here's the platforms in order of future popularity:
1) iPhone
It has three moats anyone has to cross to catch up - accessories, apps, and hardware add-ons
Accessories for the iPhone and touch can be found all over. They have iPhone/iPod docking radios in many hotel rooms now...
Apps of course, Apple has a deep lead in - but the lead is greater than it would appear because in addition to all of the apps, there are now tons of developers with iPhone experience. Sure some of the initial apps are meaningless fluff but there are now countless categories with deep teams of people working on real applications.
The last moat is new but no-one is close enough to doing anything similar that they can prevent it - and that's the ability with 3.0 for companies to build specific devices and talk to them over bluetooth and the dock connector. The TomTom car iPhone dock (more than just a dock, it also houses a more powerful GPS reciever the TomTom app can make use of) is the tip of the iceburg in this regard. Medical devices, analysis tools, etc.
2) Palm Pre
The Palm Pre has the advantage that a lot of Palm users that really want to stick with them have a great upgrade path, and the Pre has the same advantage Apple has in that they are building a single device themselves that makes it easier to build an application that takes full advantage of the platform and really fits in well with the philosophy of the device. Only the more limited nature of the SDK is preventing a much larger selection of apps, but I think we'll see this platform grow pretty well. The only danger is, I can see them being bought out my Microsoft and then they are toast.
3) Android
Android has the advantage of more devices. But that also makes it a lot harder to build and test apps, and to write apps that take best advantage of specific features. I think there will be more physical devices around than the Pre but I'll bet in three years there's more application use going on with the Pre than the Android platform.
For ease of development, I'd rate the Pre first (unless you are not very familiar with CSS), the iPhone second, and Android third (simply because not as many resources exist/will exist to help you with Android development, it has about as large a framework set as the iPhone to make things easy for you).
Symbian and Windows Mobile are simply out of the game. Unless they buy one of these three they are done as a serious platform for mobile software, despite how many devices may be around right now (see: Android issues regarding device range). They also just don't have either the framework or IDE richness these three platforms have and are growing every day...