UISearchBar within a UIToolbar won't show cancel button

I have a UISearchBar nested as a UIBarButtonItem within a UIToolbar. The basic functionality works, but the cancel button and the scope bar refuse to display. I've tried enabling them in the interface builder, and I've also tried manually calling [searchBar setShowsCancelButton:YES], but neither method works. Any ideas?

This is on an iPad. I tried in iOS 3, and it doesn't work in 4.2, either.