Did you ever found that the "lightDirection" attribute of LightInfo node never update automaticlly when you change the light direction in the scene. Following is what I done:
1. first connected the "worldMatrix" of light (say light1) to the "worldMatrix" attribute of LightInfo.
2. then I connected "lightDirection" directly to the "ambient color" of lambert, and turn off the intensity of light1. (that's mean only get what the "lightDirection" looks like).
3. I changed the light direction, but the output ambient color didn't change at the same time until I mannually reconnected the link between "lightDirection" and "ambient color"!
4. but if I connect "lightPosition" to "ambient color", it will update in realtime whenever you move the light.
Why? LightInfo prefer "position" than "direction"? I found the same problem in Maya7.0 and 8.5.
Help me, Thanks!
PS: of course light1 is not point light.