David Chappell has posted an article titled, "Services as an Explicit Abstraction"  In this article, David states that the 3-tier application arch. model of Data -- Logic -- Presentation layers has evolved with the adoption of services into a 4-tier model... Data -- Logic -- Access -- Clients.   Due to the increase of types of presantation layers, and the evolution of services that are also clients (services that might combine the outputs from other services), this (new?) layer of abstraction is being implemented today and is a better way to represent the app dev model.  Good article.  BizTalk and Indigo related...