Poking around Magento 2's core code, it looks like the old class rewrite system has been ripped out and replaced with a new dependency injection system.
Unfortunately, there's no documentation for this new system.
Does anyone have a module configuration sample that would let a user, in Magento 2, "rewrite" the class
Mage_Catalog_Model_Product with a new class
Packagename_Modulename_Model_Product, using the new dependency injection system?