I am guessing you would dump all pertinent information of the current state of the application and forms and what forms are being used/seen currently into a cache file in the eventhandler for the application closing/suspending. You need to look in the application object methods, UIApplication or UIWindow I would venture a guess. Then have your application look for the cache file at startup. If you close the application in a certain view or state then you could just not write out the cache file, that way the application will not find it and start a new state at application load.
Just my first thoughts on the mattter. There may be a better way. I believe, and I could be wrong as I am a new Mac user/developer from a Windows Platform, that this is how Mac applications work in OS X to some degree.