Open source has proven tremendously successful in producing innovation, collaboration and standards-based interoperable solutions. As a result open source is playing an increasingly important part of business and technology strategies. This paper discusses benefits and opportunities that open source brings to commercial telecommunications service layer development and the application ecosystem. We discuss how open source can be leveraged for different purposes and in different ways: as a complement to standardization, as a way to drive and leverage innovative ecosystems, to enable increased efficiency in commercial R&D, and not the least how it opens up opportunities for increased and improved R&D collaboration between vendors and technology customers. However, we will also touch on the other side of the balance: what happens when this approach meets the classical development project ways of working. We conclude with a case study of a development project of a commercial IMS/IN application and service creation platform that was done based on and in partnership with an open source development project.