It is in the Refinery::User model. There's a before_validation filter that downcases usernames:
...
before_validation :downcase_username, :strip_username
...
private
def downcase_username
self.username = self.username.downcase if self.username?
end
You could decorate the Refinery::User model:
Refinery::User.class_eval do
private
def downcase_username
self.username if self.username?
end
end