Shadow on an "invisible" plane in cycles?

I was watching a font effect tutorial by Andrew Price on Blender Guru a few days ago, and I noticed that he used a plane that only took a shadow but was otherwise invisible. I believe that when he did this particular tutorial, it was pre-Cycles, so I was just wondering if there was a way to do that with Cycles. I wanted to do an effect where you could see a shadow like on the ground but not have the plane appear in the actual render.

Thanks!

This has been discussed not 24 ago on #blender. Bottom line is, there is no easy solution for “shadow only” material in Cycles, but this was proposed: http://www.youtube.com/watch?v=JTk9ozZZxDM

I wasn’t planning on compositing this to another image… this time, anyway, but that’s helpful to know. Thanks Stan.