<p>I am wondering what makes sense in relation to what objects to expose through a WCF service - should I add WCF Serialization specifications to my business entities or should I implement a converter that maps my business entities to the DataContracts that I want to expose through my WCF service?</p>

<p>Right now I have entities on different levels: DataAccess, Business and Contract. I have converters in place that can map entities from DataAccess to Business and from Business to Contract and vice versa. Implementing and Maintaining those is time consuming and pretty tedious. What are best practices in relation to this? </p>

<p>If I were using an OR/M such as NHibernate or Entity Framework should I be exposing the entities from the ORM directly or should I abstract them the same way I am doing now?</p>

In general, I think from a best practices standpoint, you should not expose the structure of your business objects as data contracts, but rather define "data contract-specific" classes and convert Business to Contract.  It may require extra work, but from a separation of concerns and protection from change standpoint the extra work is probably worth it.
The
Microsoft patterns & practices "Service Factory Modeling Edition"
implements this, and also provides tooling to auto-generate Business <=> Contract converter classes -- it's an excellent VS add-in, and also represents Microsoft's best practices for WCF.