If you are meaning "wrought iron" fencing (black iron look fence), I've done it a few times - in both black, white and brown. Instructions are below:
Edible Re-bar for Fencing
Edible Re-Bar! This is the coolest thing. Take a full bag of frosting with a #5 round tip. Take a long stick of spaghetti and stick it through the tip, as far as you can into the bag of frosting. Pull the spaghetti right back out (which is now coated with frosting) and lay on wax paper to dry. You can cut this (or break it) into desired lengths. The spaghetti acts as re-bar providing reinforcement. This is a great idea for fences. Color the frosting black/gray for the look of wrought-iron
You can also visit
http://www.iwanttobuildagingerbreadhouse.com/gingerbreadlandscaping.html
Or see photo's at
http://www.flickr.com/photos/ultimategingerbread/2342416062/
or
http://www.flickr.com/photos/ultimategingerbread/2341346377/
www.ultimategingerbread.com