There are lots of debates and information about developing n-tier application using Microsoft .net framework. Every one has its own mechanism. I found a interesting article at Microsoft site. It is very good and with the source code.
Here are the link for that article:
http://www.microsoft.com/belux/msdn/nl/community/columns/hyatt/ntier1.mspx
It explains every aspects of developing n-tier application with Microsoft .net framework.
Happy Programming