The game uses pixel art which doesn't really scale except by set amounts so yes we up the resolution that the game is being displayed but as the pixels are the same size it does not really look much different to the non retina version.
There are some differences depending on the device you use but most are to do with physical scale of the display rather than pixel density of the display. For example there is a lot more visible map on the iPad compared to iPhone. On iPhone we concentrate on what's important and cut the map edges to leave the interactive elements larger on the screen. On non retina iPhones we did new sprites for when the game is zoomed out so there was some work from a display perspective too.
Having said all this I don't think there is anything of note that is visually different between the retina and non retina versions on iPad.