Yes, perhaps when all those devices exist you will be able to write an application that runs on all of them, specially if you write an application that works on

Yes, perhaps when all those devices exist you will be able to write an application that runs on all of them, specially if you write an application that works on the oldest version of the OS as right now new phones are being released with widely different versions of Android. If your application does not try to match the particular look of a phone as each company is customizing the OS in a different way. Each one of the platforms you mention are tiny in the application workspace right now. Palm is just about going under, Android is widely heterogeneous in implementation by manufacturers. If you are writing a game,the interface bit might not get in your way as games generally do not match the OS anyway. All of this describes a very fragmented market. Now I will call your attention to a situation when many companies had their own implementation of what many considered a more powerful OS and one company had a simpler to use OS with a much larger number of applications. Hummm... that happened in the 90s with Unix and Windows. Well, iPhone OS looks a lot better now than Windows did then.