Hi,
Well, this is my personal experience while using Linq-To-Sql. I had 3 class libraries in addition to a web-application. In the first class library I defined my DBMLs, in the second one, business logic and Data Objects(classes which connected to DBMLs) and in the third one, my asp.net forms and code behines. The last class library was assigned to Common classes and utilities. That was a very simple but effective architecture:
1- DAL--> DBMLs
2- BAL--> Connect to DBMLs and application logics
3- Common--> Utility classes
4- Web application
I hope it helps,
Cheers