The most active and popular cast-iron based group is the Dutch Oven Cooking group on Yahoo: http://groups.yahoo.com/group/dutchovencooking/
Blatant Plug Department (grin): Cast Iron Cooking on Facebook just surpassed 500 members: http://www.facebook.com/pages/Cast-Iron-Cooking/217078104969894
The Wagner and Griswold Society forum has the biggest group of knowledgeable hardcore experts on cast iron around: http://www.griswoldandwagner.com/cgi-bin/yabb/YaBB.pl
Dutch Oven Blogs
- This blogger cooked one meal a day for her family in a Dutch Oven and wrote about each one.
- Another blog with Dutch Oven recipes and results.