<p>V8's documentation explains <a href="http://code.google.com/apis/v8/embed.html#dynamic" rel="noreferrer">how to create a Javascript object that wraps a C++ object</a>. The Javascript object holds on to a pointer to a C++ object instance. My question is, let's say you create the C++ object on the heap, how can you get a notification when the Javascript object is collected by the gc, so you can free the heap allocated C++ object?</p>

In general, if a garbage-collected language can hold references to resources outside of the language engine (files, sockets, or in your case C++ objects), you should provide a 'close' method to release that resource ASAP, no point waiting until the GC thinks it's worthwhile to destroy your object.
it gets worse if your C++ object is memory-hungry and the garbage-collected object is just a reference: you might allocate thousands of objects, and the GC only sees a few KB's of tiny objects, not enough to trigger collection; while the C++ side is struggling with tens of megabytes of stale objects.