Comparing default-constructed iterators with operator==

Does the C++ Standard say I should be able to compare two default-constructed STL iterators for equality? Are default-constructed iterators equality-comparable?

I want the following, using std::list for example:

void foo(const std::list<int>::iterator iter) {
    if (iter == std::list<int>::iterator()) {
        // Something
    }
}

std::list<int>::iterator i;
foo(i);

What I want here is something like a NULL value for iterators, but I'm not sure if it's legal. In the STL implementation included with Visual Studio 2008, they include assertions in std::list's operator==() that preclude this usage. (They check that each iterator is "owned" by the same container and default-constructed iterators have no container.) This would hint that it's not legal, or perhaps that they're being over-zealous.