Atlas issue... hm. That might be a texture issue, not an atlas issue. What are the dimensions of your atlas? Make sure it's square. I know some mobile devices don't like rectangular assets. You can force the generation of a square asset in NGUI's atlas maker tool when using the custom packing mode.
You can use a UITexture or a UI2DSprite -- neither of them use NGUI's atlases. Simplest test case is ALT+SHIFT+T to create a UITexture, assign it a texture to work with and hit Play.