I think, I heard term "Serving Architect" from a ThoughtWorks presentation. It can be described this way.
Somebody with an experience works with a team, eliminating bottlenecks and helping them to move forward in the project. Ideally he or she would have experience of multiple roles in a software project: architect (software designer), developer, tester, lead and manager; with technical hands-on expertise covering stacks used by the team. This way a serving architect can help and share experience with any role in the team, without interfering with the team dynamics.