I had the same need, but could not find a proper way to spawn the calculator and return to my application afterwards. So instead, when the user hits the calculator button, I link my application to the calculator button before letting the OS handle the keyDown event, so that when the user is done with the calculator, hitting the calculator button again relaunches my applications, where I restore my previous context (stored in my application preferences), and reset the calculator button to its original setting (kept in my application preferences).
Create a Notification to handle sysNotifyAppQuittingEvent and switch back to your application.
:: marcelo alves | xfer :: http://xfer.info - Weblog, Palm & Game Development (pt-br version only) PalmOne Tungsten|T3 + SonyEricsson Z600
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