How to use presentModalViewController to create a transparent view

After iOS 3.2 there is a method to do this without any “tricks” – see the documentation for the modalPresentationStyle property. You have a rootViewController which will present the viewController. So here's the code to success:

viewController.view.backgroundColor = [UIColor clearColor];
rootViewController.modalPresentationStyle = UIModalPresentationCurrentContext;
[rootViewController presentModalViewController:viewController animated:YES];

With this method the viewController's background will be transparent and the underlying rootViewController will be visible. Please note that this only seems to work on the iPad, see comments below.

Your view is still transparent, but once your modal controller is at the top of the stack, the view behind it is hidden (as is the case with any top-most view controller). The solution is to manually animate a view yourself; then the behind-viewController won't be hidden (since you won't have 'left' it).