Research in Motion BlackBerry phones support the Java ME standard, so for a Java ME developer is easy to get started on BlackBerry. But a few things do not work as expected...
The BlackBerry has a very consistent user interface, with a menu key and a back key. These keys can not be catched by your Java ME canvas, pressing them will not call the keyPressed() method as you might have expected. Instead the menu key will always open the menu. If you have not added Java ME commands to the Canvas, the BlackBerry will create a menu itself, with 'Close' as the only alternative. The back key will trigger the command with the command type EXIT if there is one and do nothing is there is no such command.
This basically means that you should always use Java ME Commands in yor BlackBerry application. There is no other way within the Java ME API to get the BB keys to work as expected.
There are no Nokia-style softkeys on a BlackBerry keyboard. But there is a trackball (trackwheel on older models) and a qwerty keyboard.
The trackwheel sends Canvas UP, DOWN, RIGHT and LEFT codes and trackball press can also be handled and expected. Your application should probably use these keys a lot. Key repeat is however not supported, so if the user scrolls the trackball you will get a series of keyPressed events and no keyRepeated.
The qwerty keys will call keyPressed with codes as expected, you can use getKeyName. Just remember that some models have a smaller keyboard, with two characters per key. You will always get the first character.
The return, backspace and space keys will also deliver key events as expected. The shift and alt keys will however not. Instead they affect the keycode delivered to the application. That is ok for the qwerty keys, but also it means that you can not tell if the user is holding down shift and scrolling or pressing the trackball.
You Java ME Canvas will always cover the full screen. The call to setFullScreenMode has no effect. This also means that the sizeChanged method will never be called. So you should make sure that your application does not depend on it beging called.
The conclusion is that the BlackBerry Java ME implementation is a good one, and can be very well be used as a base for your implementation on BlackBerry. But if your midlet is based on the Canvas class and developed for Nokia-style phones you will probably need to make some changes, mainly with menues, softkeys etc. You can them detect at runtime if the midlet is on a BlackBerry or not and adjust to that.