Website, DB, & Software Developers

Since you are developing in .Net and most likely a SQL Server backend, you should really look into LINQ-to-SQL and LINQ-to-EF (Entity Framework). If you are looking for a way to encapsulate business logic in one layer and data access in another, you could easily implement this in either a single or multiple assemblies. The number of tiers you implement all depends on the complexity of your needs.

This link provides a good overview:
http://msdn.microsoft.com/en-us/library/cc161164.aspx

Exposing your data through WCF makes sense if your need to consume the data from multiple clients or applications. WCF makes it very easy to build up your data contracts and manage data from typical clients via REST, SOAP, JSON, etc.. and it is very easy to build it as a self-hosted Windows Service, no need to be hosted by IIS.