This reminds me of the GPU technique to generate Voronoi diagrams by rendering conical geometry... And, sure enough, it looks like someone else already thought of using cones to generate distance fields:

http://gamma.cs.unc.edu/DIFI/Sud-EG04-DiFi-Presentation.pdf

The technique in the article is similar, but uses brightness and the minimum blending operator instead of depth -- which has the advantage that you can use it to generate distance fields over surfaces other than the plane, which is pretty cool.