While digital virtual worlds have been used in education for a number of years, advances in the capabilities and spread of technology have fed a recent boom in interest in massively multi-user 3D virtual worlds for entertainment, and this in turn has led to a surge of interest in their educational applications. In this paper we briefly review the use of virtual worlds for education, from informal learning to formal instruction, and consider what is required to turn a virtual world from a Multi-User Virtual Environment into a fully fledged 3D Virtual Learning Environment (VLE). In this we focus on the development of Sloodle--a system which integrates the popular 3D virtual world of Second Life with the open-source VLE Moodle. Our intent is not simply to provide additional learning support features for Second Life, but to study more generally the ways in which integrated virtual environments can benefit teaching and learning, and this is the focus of our closing discussion. (Contains 6 figures and 2 notes.)