If there’s a better way to do this, I’ll eat my code.

UITextFieldDelegateObject.m

- (BOOL) textField: (UITextField *) textField shouldChangeCharactersInRange: (NSRange) range replacementString: (NSString *) string
{
	([(textField == self.serverField ? self.emailField : self.serverField).text length] > 0 && 
	 [(textField == self.passwordField ? self.emailField : self.passwordField).text length] > 0 && 
	 [[textField.text stringByReplacingCharactersInRange: range withString: string] length] > 0
	 ) ? (self.navigationItem.rightBarButtonItem.enabled = YES) : (self.navigationItem.rightBarButtonItem.enabled = NO) ;

	return YES;
}