I am learning and working with a Business Objects framework at work called CSLA.NET (Component-based Scalable, Logical Architecture).  It is built heavily using the best of OOP especially polymorphism and abstraction.  You build your objects all based on a handful of base classes that provide a huge variability of useful features.  It also abstracts out data and .NET remoting layers to make the developers life easier.  If you are building large, scalable n-tier applications, you should definitely look this one up. 

Also check out the book Expert C# 2005 Business Objects, which is the companion to the framework.  If nothing else, it contains a great description of n-tier architecture and why to use it.