Nested closure is one of those fancy patterns Martin Fowler first coined (as far as I know) and published on his DSL WIP site. His formal definition:
Express statement sub-elements of a function call by putting them into a closure in an argument.

You pass a delegate as a method parameter. The receiving method executes the function [...]

Some folks on the nh-users list (led by James Gregory) have moved Jeremy Miller’s Fluent NHibernate project to Google Code.
There’s a lot of work to do, but we have a list going.  Should be an interesting project to watch (and to contribute)!