Arrowroot can come from several different food families. Some on the market are "cut" with cheaper starches like potato or corn. According to Bob's Red Mill their arrowroot "is the fecula (starch) which is separated from the root of Maranta Arundinacea, a native plant to the West Indian Islands and sometimes Central America. It is not derived from plantain, but rather an herbaceous perennial".
Other sources of arrowroot are:
Dasheen Arrowroot in the Arum food family
Florida Arrowroot in the Cycad (sago) food family
East Indian Arrowroot in the Ginger food family
Brazilian arrowroot is the source of tapioca and in the Spurge (cassava plant) food family
Queensland Arrowroot in the Canna food family
Update: Bob's Red Mill has been found to be corny, so if you are avoiding corn, you'll want to find a different brand. As of August 2012, we are using Authentic Foods Arrowroot flour. According to the manufacturer it is corn-free. So far it's working for us.