If you've upgraded to South 0.7, you'll notice that custom model fields are no longer supported.
There's a long, convoluted, discussion about supporting custom fields with introspection rules, but that's unnecessary for most custom fields. If you're just extending a standard field like CharField, follow their tutorial example.
from south.modelsinspector import add_introspection_rulesin your fields.py file (which should be holding the custom field MyCustomField, in my case in the util app).
Then, at the bottom, under your field definition, put: