A couple of days ago while working on a design for an iPad application I came across the situation that I wanted some interface elements to be the size of some real world objects. Moments later I found myself using a quickly created image that helped me to get a better understanding pixel/cm ratio.
Sure, I could have just calculated the values but it just felt more natural to arrange actual things at the screen. If you’ll ever find yourself in a similar situation feel free to use the image. I’ve added a png file at the iPad’s native resolution (1024×768) as well as the pdf, in case you would like to modify it.