I'm experiencing this exact behavior using STI
Are the subclasses in the same file as the base class? If not, 'require' or 'load' might do the trick.
Dude man! Thanks SOOOOO MUCH. I added require_dependency 'model' see the bottom of http://wiki.rubyonrails.org/rails/pages/SingleTableInheritance - DOH! if only I read that more closely.