Is it possible to implement a recursive "SelectMany"?

As we all know, Enumerable.SelectMany flattens a sequence of sequences into a single sequence. What if we wanted a method that could flatten sequences of sequences of sequences, and so on recursively?

I came up quickly with an implementation using an ICollection<T>, i.e. eagerly evaluated, but I'm still scratching my head as to how to make a lazily-evaluated one, say, using the yield keyword.

static List<T> Flatten<T>(IEnumerable list)  {
    var rv = new List<T>();
    InnerFlatten(list, rv);
    return rv;
}

static void InnerFlatten<T>(IEnumerable list, ICollection<T> acc) {
    foreach (var elem in list) {
        var collection = elem as IEnumerable;
        if (collection != null) {
            InnerFlatten(collection, acc);
        }
        else {
            acc.Add((T)elem);
        }
    }
}

Any ideas? Examples in any .NET language welcome.