Programming to interfaces while mapping with Fluent NHibernate

Question

I have been whipped into submission and have started learning Fluent NHibernate (no previous NHibernate experience). In my project, I am programming to interfaces to reduce

Answer 1

Looks like I don't have enough reputation to comment on other peoples answers yet as such I'm going to have to make this an answer in it's own right.

References now has a generic overload to allow the cast that theGecko was looking for in his answer.