Updated: 22 May 2012 

I’ve done some research into the different Mobile Development Platforms, as there’s now so many different offerings from different manufacturers.

Here’s the current state of play:-

iPhone / iPad 
Objective-C using iOS development platform

Java using Android SDK development platform

Java using  BlackBerry API development platform – BlackBerry Java SDK
Can also port Android apps to BlackBerry using BlackBerry Packager for Android Apps

Amazon Kindle Fire
Uses customised version of Android OS 2.3

Sony Tablet 
Uses Android 3.1 Honeycomb

Samsung Tablets
Uses Android OS, also Samsung libraries, plus also their own Bada OS, which uses C++

All Platforms – HTML5/WebKit
Using HTML5 would allow web apps to work on all of these devices but – in most cases – you wouldn’t be able to access any of the specific hardware of each device. This may change over time, if the relevant support is added somehow.

Porting Android Apps 
From the above list, this means that if you develop an App in Android / Java you can then port it to BlackBerry and the Amazon Kindle Fire. I assume a port to the BlackBerry would take more effort/time than a port to the Kindle Fire.