Yet another awesome thing. How to link a 2D point to a 3D point (like a light or a 3D null) and it's hella simple. Find the tutorial I got this from Video Copilot. You know who they are... and if you don't, PLEASE check out their large library of tutorials.
1- Alt + Click the 2D position value
2- Pickwhip the 3D layer then add ".toComp([0,0,0]);"
If you've used After Effects for a small period of time, chances are you have come across the problem of trying to connect a 3D layer position with a 2D effect (like trying to connect the standard lens flare effect to a 3D null or something). Well this helps you solve that! Plot this expression in the 2D point and you're good!
I've found, however, that you can modify this expression to connect a 2D point with another 2D point! This works really well when you want something to follow something else (like maybe a puppet pin point to a controller null or something). Simply take out a ",0" and you'll end up with: