Turn on/off Portrait Orientation, Depending on the Time of Day
This is certainly not possible with the stock software provided by the iOS and I haven't seen any application in the AppStore that does such thing. Should be "relatively" easy to do, but I'm not sure who'd want a software like that (other than you) ;)

EDIT: Now that I think about this, I'm not sure if the API for controlling that is public or private. If the later, then you can't really create an app that "modifies" the Phone's Settings, so my best guess is that you'd be out of luck.