LINQ to SQL Common Base Class Problem

f you are a fan of LINQ to SQL, you may remember Dinesh's post where he mentioned the possibility to create and use a common base class for all your entities. I really like the idea, and to clarify, I have been using it in one of my projects even before seeing his post regarding the subject. But unfortunately, yesterday, while playing with a piece of code, I happened to face a problem that I want to describe in this post.