Billy McCafferty has a nice series on collections in NHibernate. I like his discussion of good places to put particular retrieval methods (FindAllByDateRange is his example). His thoughts on separating out responsibilities between the data access layer and domain layer are thought provoking and well-reasoned.
McCafferty walks through some various scenarios of where such methods could live and lays out good pros and cons along the way. His article's really about how you can work solve problems around collections, but the general point of where and how to implement accessors is an important one too.