I'm also interested in this issue, since I've just the same 'structure'. (A dictionary containing another dictionary).
However, I've worked around this, by creating another class, which holds the other List.
So, instead of this:
private IDictionary<int, List<string>> _elements;
I have done this
private IDictionary<int, ItemCollection> _elements
where the ItemCollection class, contains another List.
But now however, I experience another problem with this; when I run my program, NHibernate complains that it could not compile the mapping document.
It says 'error mapping generic collection ... expected one generic parameter, but found 2' :(