One of the things I like with NHibernate is that my domain model needs absolutely no knowledge of the data layer - all of it is described in separate xml files.

But if I want to use a custom data type, say an Address (roadname, postal code etc.) then my domain class for such an object must implement IUserType - thereby adding a dependency from my domain layer to the data/persistence layer.

Is this correct or have I overlooked something?

Not necessarily. You can implement the Address class as part of your domain layer and AddressType implementing IUserType in the data access layer of your applicaction. Then you can map a property of type Address with AddressType in the mapping file.

Thanks, I will try that.