From the big picture perspective, SharePoint Server 2007 can't really be understood apart from the architecture of the 2007 Microsoft Office system. With the release of the 2007 Microsoft Office system, Microsoft created a logical architecture plan that makes use of the best concepts in architecture planning available today. Each service is intended to stand on its own as an independent software component. These services are grouped into cooperative sets that then form applications within the Microsoft Office system. SharePoint Server 2007 is one of the core applications of the 2007 Microsoft Office system. By building all the applications out of the same set of services, Microsoft is driving toward universal compatibility among all of its products and applications. Using a common set of services within Microsoft Office system as application building blocks drives those applications toward simplification and compatibility.
Keep in mind that Microsoft's architecture plan is still a work in progress, as software always is. Microsoft is pursing compatibility among all of its applications, but it isn't quite there yet. The 2007 Microsoft Office release is a giant leap toward this goal, but it is not the whole journey. It is, however, a wonderful leap.