Ive just uploaded a small .NET 2.0 demo of NPath.
The demo uses NPath to sort and filter a generic list of custom ("Person") objects.
You can use this with any .NET class so NPath is not limited to only NPersist, you can just as well query a graph of NHibernate objects or whatever you like 
You can download the demo from:
http://www.puzzleframework.com/tmp/npathdemo.rar
In mem query engines have some nice benefits:
* You can offload your database (especially on fat clients)
* You can query graphs of objects that have not yet been saved to a DB
* You can query graphs of objects that doesnt have anything to do with a DB at all
//Roger