*Pixel* padding. Sry...When the edge pixels are extended out. When you don't have this, then you'll get such seam line errors once the texture gets mipped.
You can set your edge padding when you pack your unwrap. It's the amount of pixels between each UV shell. This is to prevent edge bleed between UV islands for both baking and mip-mapping.(1 pixel=1 mip level up to 16 pixels=5 mip levels)