UINavigationController crashes in 4.0 - not <-4.0.

Question

I'm trying to push a UIViewController with UINavigationController. It's working perfectly in any SDK prior to 4.0, but it just won't work in 4.0.

I'm getting the OBJC_MSG_SEND (EXC_BAD_ACCESS) error. The UIViewController isn't nil, neither is self.navigationController.

I've tried NSZombieEnabled, but nothing shows up.

The UINavigationController is running under a UITabBarController as one of the views to the controller. It's all been setup in MainWindows.xib.

Any ideas for what is wrong? It's driving me nuts.