The really exciting Java applications and services, including games, will be those that bring together a wide range of resources and bring people together. So a sales application might use a product catalogue, route planning, a time table, and location services. Or Java can add value to even a simple game like "Snake" by allowing me to play against my children when I'm overseas with a few minutes between meetings.
The PersonalJava and JavaPhone APIs on Symbian OS provides perhaps the richest Java environment for mobile phones. As an example we might use this capability to create an application for coordinating a meeting: it would use calendar APIs, contacts APIs, and messaging APIs to negotiate a suitable time and venue amongst the delegates.
Technical reasons to do with:
Your registration key can only be used on one device at a time. Should you upgrade or change devices, you can enter your registration info when prompted by the device. The key transfer between your old device and your new one will happen automatically.