we are actually using XNA 3.1 as well.
and this example for different screens http://create.msdn.com/en-US/education/catalog/sample/game_state_management, 3.1 version.
we couldn't get it to work, and have been working on it over a month.
so decided to separate games, and get rid of the menu.
but if you have any other suggestions, i would like to hear. we still have the crashing version. if it simply changing couple of lines, where we messed up, that would be awesome.
the way we set it up right now is, each screen contains a Goblin scene. and every time you go into a screen you create a new Goblin scene.
the problem comes down to i think with AlvarMarkerTracker.cs. As I said before, if the camera doesnt see a marker, we can switch back and forth between screens with no crash. But after this switching back and forth is done like 3 or 4 times, and the camera
sees a marker, the program crashes. I think the AlvarMarkerTracker is not freeing a source that it is using.