When a window or dialog is opened which has an icon, after it is closed it cannot be garbage collected because a reference is retained to it through the icon. Removing the icon resolves the issue.
As far as I've investigated, the issue appears to the in the ImageListenerList which each Image contains. I've done a heapdump of my application and used the IBM HeapAnalyzer which shows this list containing a reference to the window/dialog through the ImageViewSkin, but I don't know the Pivot internals well enough to see where or how it should be released.
I'll attach a simple test application to show the issue.