If you're an NHibernate user, and/or interesting in learning about how you can create custom implementations of LINQ using IQueryable, I strongly urge you to checkout what Ayende and Bobby Diaz (and others?) have been up to with LINQ to NHibernate.

Here are some of my favorite posts thus far:

• Implmenting Linq for NHibernate: A How to Guide Part 1
  
• Implementing Linq for NHibernate - Part 2 (Ordering and Paging)
• Implementing Linq for NHibernate - Part 3 (Aggregate and Element Operators)