In the MoSaiC project (Mashups for Situational Collaboration), we propose to investigate on a novel service-oriented approach for composite enterprise documents supporting ad-hoc collaboration and weakly structured innovative processes within and across enterprises. As technical foundation, we propose to integrate electronic documents with software services and processes underlying SOA. We are developing a Web-based architecture and prototype platform of a lightweight document service bus that shall represent document fragments as stateless software services and introduce a simple composition model to coordinate document content, editing and publishing services. Our document service model will allow us to conduct exciting research on an intuitive mashup language for document-centered situational collaboration. Collaborative mashups will regulate and enforce flexible and dynamic interaction patterns for document communication supporting conventional and virtual teams.