Here’s a nice article I found in about the different approaches of the different mobile OS.

The article talks about iPhone being a completely closed system, giving the software developers the benefit of knowing exactly the target OS and the on the exact devices.
This is the same concept RIM created with the Blackberry (although few different devices instead of 1).

On the other extreme – Google’s Android – a completely open operating system. This will give unlimited flexibility, and since the source code will be available – companies will even be able to create sub versions of the android if they wish.
This is the strength of the Android, but can also end up as a problem, when talking about new versions, causing a variety of problems, limitations and modifications…

In the middle, we have Microsoft’s Windows Mobile and Symbian.
Both are not open source, but they have many different capabilities which are being implemented differently per different devices manufacturers (HTC, HP, Samsung). And this is why you can see different features implemented differently between HTC and HP (for example).
Palm, Inc. is following RIM and Apple and is in the process of creating a proprietary Linux-based operating system for its consumer-oriented smartphones. Generally called either Palm OS II or Nova , this will debut next year.
Palm is also creating Windows Mobile devices such as the Treo products.

Nokia and the whole group of companies that use the Symbian OS are going the opposite direction by taking Google and the OHA's path. The Symbian OS is going to become completely open soon.