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.
Mike K's answer is false.
While mixing XIBs and Storyboards doesn't really solve Alex's problem, it is quite possible to do within the same project.
See: https://stackoverflow.com/a/8516479/1309238 re: loading a second storyboard.
I'm not sure if including a "Main Storyboard" in your Target Summary is advisable if you plan to do this, but if you've loaded a XIB instead of a storyboard to begin with, you can modify the code linked above to load the storyboard after the XIB.
I'm not sure if you can jump into the middle of a storyboard from a XIB, which may have been what Mike meant.