You are correct that the 9.x Cell Tool developer sample does not work in 10.x. Few 9.x ArcObjects apps do, as the entire framework changed a lot at 10x.
If you need to see cell boundaries a method I recommend is to create a random integer grid and display it transparent using the discrete renderer.
Also, there is a 10.x tool (though you need to add the button to the interface using Customize) that is maybe exactly what you are looking for - it's called Pixel Inspector.
