See this commit from slicer3 for implementation ideas on the warn before exit:

http://viewvc.slicer.org/viewvc.cgi/Slicer3?view=revision&revision=15607

As noted above, a complexity is deciding which changes to nodes should be considered 'modifications'. One possibility would be to explicitly require the user to specify a scene file when the program starts - something like Xcode where most menus are disabled unless there is an active project selected. In this case we could check the MTime of all nodes and consider that any modification should be saved. It would be a change in behavior and I don't know how users might react to it.

I am note sure we want to ask the user to provide name each time the program starts, instead when slicer is open, the current scene could probably be named "Untitled scene", if the user doesn't do anything .. it shouldn't ask him to save. If later we decide it's better to ask the user to provide a name each time the program starts .. it should be trivial to add.

If the user modify the scene in some way (either through the user interface or using python script .. ), it should show a small "star" near the name to indicate the scene has to be saved / is modified. Considering the name is "Untitled", this save action should be like a "Save as..." .. the subsequent call to save should be like normal "Save".

A side feature:
The concept of auto-save could also probably be integrated .. so that every X minutes, the scene is auto-saved, This should be optional.

Fixed in r20360:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=20360

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