The JISC e-Learning Programme has released Serious Virtual Words: A Scoping Study.
Here's an excerpt from the announcement:
The aim of the report then is two-fold: to provide a context for learning practitioners and policy makers, aiding with their understanding of virtual worlds and how they can be selected and used in tertiary education; and to highlight how learners, through greater empowerment, may play a different and enriched role in the process of forming collaborative learning experiences and engaging in activities which may support their own learning and meta-reflection.