If the repository component can persist things in memory, we would not have a need for mocking IQueryable, an example:
Here's the repository component I made that works on memory, NHibernate, Entity Framework:
Other's approach on unit testing without using repository component + mocking is by using an in-memory database like SQLite, this precludes the need for repository component:
I ceased the development of my repository component project as it is hard to abstract the differences of fetching strategies between NHibernate and Entity Framework:
I tried many ways to make the repository component conform to Entity Framework's intuitive fetching API, but it's hard to make NHibernate's fetching strategy similar to Entity Framework. Even I don't like Entity Framework, I'm not a technology bigot and I'm able to see some of Entity Framework's good points. So if I'm able to abstract the fetching strategy on the repository component, it would be patterned after Entity Framework's fetching strategies, i.e., not on NHibernate's buggy ThenFetchMany nor on NHibernate's ToFuture. Entity Framework's fetching strategy is easier to use and intuitive, It Just Works.
Here's what can happen if the API can't empower the developer, it compels them to use N+1. A case of too much abstraction: http://ayende.com/blog/156577/on-umbracos-nhibernates-pullout