You don't have Autolayout in iOS5, but you do still have the autoresizing mask. If you center a view vertically in IB, go to the inspector and deselect the vertical struts and springs in the autolayout masks, the view will stay centered vertically on both 4" and 3.5" screens.
Or you can do this in code in your viewDidLoad method. Just center the view as you are now and set the autoresizingMask like this:
self.customButton.autoresizingMask = (UIViewAutoresizingFlexibleBottomMargin | \
UIViewAutoresizingFlexibleTopMargin);