Harry previously mentioned the expression in Harry Frank's 5 Favorite Expressions and in a video tip for making a 3D volumetric light with Trapcode Shine (and probably in his training products).
This layer space transform expression was also popularized by Dan Ebberts in his 3D lens flare case study, by Andrew Kramer in a few tutorials (see Lens Flare on 3D Lights?), and by Chris & Trish Meyer in their series on expressions.
Update: there's also a Vimeo version,
Using the toComp Expression from graymachine on Vimeo.