John Evdemon (a solutions architect with the Microsoft Architecture Strategy Team) just published 2 articles on the principles of web service design.
Principles of Service Design: Service Patterns and Anti-Patterns (which I posted already a while ago)
Summary: This paper, the first of a multi-paper series, provides some fundamental principles for designing and implementing Web services. The paper starts with a brief review of SOA concepts and leads up to a detailed discussion of several patterns and anti-patterns that developers can leverage when building Web services today. Guidance in this paper is applicable to any programming language or platform for which Web services can be developed and deployed. This paper includes three working examples that you can download and review.
Summary: This paper is the second in a multi-paper series focusing on service design principles. This paper surveys some fundamental guidelines for versioning schemas and Web services. Six specific principles for versioning Web services are identified.
And John is running a interesting blog on Service Design.