The most active and popular cast-iron based group is the Dutch Oven Cooking group on Yahoo:

Blatant Plug Department (grin): Cast Iron Cooking on Facebook just surpassed 500 members:

The Wagner and Griswold Society forum has the biggest group of knowledgeable hardcore experts on cast iron around:

Dutch Oven Blogs

Dutch Oven Madness

This blogger cooked one meal a day for her family in a Dutch Oven and wrote about each one.

Cooking with Gilman

Another blog with Dutch Oven recipes and results.