I did in fact spend a lot of time deliberating how to use LINQ in my repository framework, and I came to the conclusion that you could use it if you wanted to behind the walls of the repository interface. Basically, it is not part of the framework, but any repository can use whatever it wants to get at the data it needs. I thought about making a generic repository that would take in LINQ lambda expressions for doing selections, but I did not feel that it was worth the effort.
I wanted to use the ADO.NET Entity Framework, but I really hated using it. LINQ to SQL was even worse. So, I just kind of threw in the towel and went with what I had, in hopes of changing it to use the EF once it got a lot better.