One I was thinking of is almost like a 'virtual window' where you hold your device up and it shows you what ever geo overlayed information you want. Obviously this would work best if the device can tell what direction it is facing, what its orientation is etc. You could pan the device across your surroundings to see old pictures, information on buildings and more active things like how to book, tourist information etc

 I like it as it gives you a virtual window to see whatever additonal layer of information you are interested in and would work in a very intuitive way. I heard about an iPhone app which helps with identifying constelations in the night sky so it sounds like we're nearly there.