Tengo que admitir que me ilucione con el session_start(); era completamente logico.... pero no, me dio el mismo error. Hasta probe ejecutarlo antes del include me dio otro error:
Fatal error: main() [<a href='function.main'>function.main</a>]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "carrito" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in ....\tvirtual2\mete_producto.php on line 4