Worth noting that you'd be writing once for the PLATFORM, but being certain that your app works on the DEVICES is a different kettle of fish due to the variety of handset providers, their customized platform implementations and the hardware specific to the device.
Check out these related questions... otherwise know as duplicates....
Developing cross platform mobile application
Are there any huge differences between objective-c and Java, or iPhone and Android?
What work has been done on cross-platform mobile development?