I have some UIViewControllers from my old iOS4 project, they are using .xib, created in interface builder.
My new project, built for iOS5, uses storyboards.
You could copy the contents of the xib into the storyboard and then instantiate it using:
- (id)instantiateViewControllerWithIdentifier:(NSString *)identifier
after setting the identifier on it. If you need the instance of the storyboard you can get it this way:
[UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];