"see through" displays are great for glasses (hmd) or car windshields, but I fear in a handheld the field of view is a problem. Much better to display an image and overlay things there. Not only you can use a a larger FOV, but you don't force the user to hold the thing right ahead of him, and you can also guarantee the artificial overlay will be properly aligned to the objects being displayed.

Think about small camcorders... Today the trend is having an lcd display that you can move around. It's much more comphortable. But I repeat: see-through AR displays have their place in the HMD world.