I want to learn some mobile stuff, and programming interesting things myself on mobile phones, what is popular and easy-to-use mobile operating system?
.Net Compact Framework is pretty easy to work with. And works great on Windows Mobile devices.
Windows Mobile has fairly easy development with .NET Compact Framework.
WebOS (Palm Pre, etc.) has an easy to use HTML/Javascript like environment. Check out their 'Hello World!' example to see what I mean.
Android isn't bad if you're familiar with Java. (Same goes for Blackberry)
iPhone might be the most complex in my opinion because of the use of Objective-C (it's also probably the most robust). Check out their Programming Guide Introduction to get an idea about what I mean. You might want to check out MonoTouch which allows iPhone development in .NET.
This is extremely subjective.
Java developers will be more comfortable on the Blackberry, Android or J2ME platforms, while Windows developers will likely be more comfortable with Windows Mobile.
Personally, I find programming for the iPhone very enjoyable and easy. I found Objective-C easy to learn. The tools are also well made and the frameworks intuitive IMO.
I've also come to personally dislike coding for the Blackberry. Simple applications require too much code and designing interfaces is an enormous pain.
But as I said, this is very subjective. I'd recommend going through the Getting Started examples of the each of the platforms you have in mind and deciding after having some exposure to each.
Without to know which platform you are used to program on, it is not possible to give a correct answer. If you are used to program on Windows, and you are already using .NET framework, then it would be better for you to learn how to program on Windows Mobile; if you are already a Mac developer, then it is easier for you to learn how to create a program for iPhone OS (which is also used on the iPod); if you are a Java programmer, then you should be more comfortable with Android.
IMO, you start to program on a mobile platform that can be programmed using the same framework for the desktop applications because that would allow you to not learn two different frameworks