You will probably want to get the difference between your Flex application's height and the device's height:
var actionBarHeight:int = FlexGlobals.topLevelApplication.height - Capabilities.screenResolutionY;
The above solution will not work when testing on the desktop as Capabilities.screenResolutionY returns the actual screen resolution of the host machine and not that of the emulated device.
I too would really appreciate a method to obtain the devices status bar height (i.e Capabilities.statusBarHeight) for calulating component width and height upfront.
In that case you'll probably want to check if Capabilities.os is Windows/Mac and return 0 for the status bar height.