I might be dumb here as I just started messing with lighting on sprites, but the tangents that tk2d seems to auto generate are (1, 0, 0, 1). Should this not be (1,0,0,-1) to work properly with normal maps that Unity imports from height maps? (The normals tk2d generates are (0,0,-1) as expected).
For example start with a grayscale height map, import it into unity, set it as a normal map and check the checkbox for gray scale. Use that normal map on a sprite with a directional light coming from the top left of the screen. For tk2d sprites the light appears to be coming from the bottom left. Now I know I can just edit tk2dSpriteCollectionBuilder.cs and change the tangents that get generated (this works, I've done it), but before I do that and move on I wanted to see if there was something I was missing first. For all I know Unity's conversion from height map to normal map is upside down.
(Note that I have the linked sprite collection atlas that tk2d generates set to Normal map, grayscale. So I'm using Unity's built in grayscale height map to normal capability).
As a side note, I just started using 2DToolkit (like 2 hours ago) and have been using Unity's straight up SpriteRenderer on my current project. I wanted to use 2dtoolkit because of the linked sprite collections that simplify dealing with normal maps and atlases. And it seems to do the beautifully. That alone makes 2dtoolkit awesome. Throw in that you can slice sprites and it's a clear winner over the stock SpriteRenderer in Unity 4.6.