I need some advice how we can decouple nHibernate dependencies in the presentation layer. Currently we have a three tier C# winforms application consisting (simplified) of the following layers;

* User Interface (UI)
* Business Logic (BAL)
* Data Access Logic (DAL)

We are migrating this application to an ORM (nHibernate) and would ideally like to have only the DAL referencing nHibernate. We also want to employ the "Unit of Work" functionality which is included in nHibernate, adopting a "Session per conversation" methodology. 

To achieve this we need to create and open a session in the UI, pass the session through the BAL to the DAL however we cannot achieve this without creating a dependency to nHibernate in both the BAL and DAL.

Any advice would be appreciated. How should we structure the architecture to avoid any references to nHibernate in the UI and BAL. Any ideas?

I must also add that we do not want the UI to have a reference to the DAL either.

UI => BAL => DAL