Can one develop an entire application using JavaFX and run it on iOS, Android or Windows Phone 8, without writing platform-specific code?
Yes you can run JavaFX application on iOS, android, desktop, RaspberryPI (no windows8 mobile yet).
Work in Action :
Dev Resources :
If you are going to develop serious applications here is some more info
At present for JavaFX Oracle priority list is Desktop (Mac,windows,linux)
and Embedded (Raspberry Pi, beagle Board etc) .For iOS/android oracle done most of the hardwork and opnesourced javafxports of these platforms as part of OpenJFX ,but there is no JVM from oracle for ios/android.Community is putting all together by filling missing piece(JVM) for ios/android,Community made good progress in running JavaFX on ios (RoboVM) / android(DalvikVM). If you want you can also contribute to the community by sponsoring (Become a RoboVM sponsor) or start developing apps and report issues.
Edit 06/23/2014 :
Johan Vos created a website for javafx ports JavaFX on Mobile and Tablets,check this for updated info ..