I've been looking into the Exception Handling Block which is part of the Enterprise Library January 2006. There is a good article on DevX which goes over defining Policies, using the Enterprise Library Logging Block and also Custom Handlers. This article also covers the same thing but in more detail. This MSDN article highlights a use for this with a Critical Errors Database scenario.
The diagram in the M$ help shows the method of defining policies for each layer and what should be logged where.