In a previous post Tracking Down EXC_BAD_ACCESS Errors with NSZombieEnabled I explained how the environment variable NSZombieEnabled can help track down EXC_BAD_ACCESS errors, which are typically caused by attempting to access objects that have already been released.
With Xcode 4 the process for setting NSZombieEnabled is different than in earlier versions of Xcode. To configure zombies, enter ⌥-⌘-r (alt-command-r). Once the dialog is shown, choose the option Enable Zombie Objects
When using a zombie, any message sent to an object that has already released will result in an exception being thrown, which will look similar to the screenshot below: