http://www.agilewiki.org/projects/utils/
Immutable map list is now done, tested, working and released. And was remarkably easy to do.
Now I suspect things will get a bit tougher as we start work on to lazy deserialization. I've seen how huge this can get, so my primary concern is keeping it small. So I am thinking that only map list, String and the boxed immutable scalars like Integer and Double will be supported initially.