It may sound stupid, but I think the best mobile platform is the web. You can't know what platform will be the most widely used in a year or two, Symbian, Android, or even that iPhone thing, but you can be sure that more and more mobile devices will get UMTS and/or WLAN and they all will have browsers. And it's pretty easy for you to develop such applications, compared to embedded Java stuff. Plus you can use them from your notebook/desktop too.
