Last year, I wrote a series of posts (part I, II and III) concerning the devlopment of custom collections which are compatible with NHibernate.  I've written a new post which greatly simplifies this process with the use of (evil) extension methods:  http://devlicio.us/blogs/billy_mccafferty/archive/2008/09/03/custom-collections-with-nhibernate-part-iv-extensions.aspx.  I believe you'll find this much simpler and more elegant than my previous suggestion.

Billy McCafferty