This is largely becoming the most common expression I use. It is so simple and so powerful.  This was originally something I posted on AE Freemart, and is based on the amazingly awesome and inspirational ideas from Dan Ebberts.

Simply put, the location of any 3D layer (Null, Light, Solid, etc) can be adapted from 3D space to a 2D location.  Imagine, the location of a 3D light driving the location of Knoll Light Factory flare, or the example below, where a 3D beam is created using two 3D nulls.

Essentially, you just need to create an expression on your 2D location, and add an expression like the one below. Replace the layer name (ie.. "Null 1"), with your 3D layer.  This is so useful!!! I use it in just about every project.

>Download the AE CS3 3D Beam Project

thisComp.layer("Null 1").toComp([0,0,0])