This evolved out of a test for a project that we didn't end up doing. The idea was to create a skull out of wafting smoke.
I ended up doing this with Krakatoa and Particle Flow, with *much* help from Bobo and the guys on the Krakatoa forum. It was a great learning experience and the end piece worked out pretty well. See the link for a higher-res version of the full spread. Original renders were about 6K high (for each skull) and the total spread was about 10-12K wide.