Steve,
Sorry I am late chiming in here. Supporting IPortable in your module means that you will implement import and export methods that will be fired when a user clicks on the "Import Content" or "Export Content" items on the module action menu.
These methods need to basically serialize and deserialize your module's data and/or settings. Usually the serialization is done as an xml file.
When copying a page or creating a page template, I believe DNN will fire your module's export function. I would need to do more testing on how the Copy module interacts with this. This is a good tutorial topic!
Steve, have you done a baseline test of your IPortable implementation? Make sure to indicate portable in the supportedFeatures in your manifest. Make sure your implementation works and debug it by clicking Export Content / Import Content on the action menu.
Once you know your implementation works. Try copying the module while your debugger is attached to see if it enters your IPortable method.