Campus technology has an article http://campustechnology.com/articles/2009/03/04/second-life-engaging-virtual-campuses.aspx describing the "ghost towns" created by various corporations and universities. It also describes the SL campuses of several universities like the University of Delaware and Montclair State University.
The question is whether the real use of SL in education is the structures and buildings of these universities or is it the ability to interact with others in a unique way? The ability to cloak oneself behind one's avatar lends a special characteristic to SL interactions.
Physicians are well aware of how some patients are able to ask questions more frankly via e-mail rather than face to face (See article by Daniel Sands http://www.mahealthdata.org/data/library/guidefiles/1999_PatientCtrdEmailGdlns-Sands.pdf). Does SL allow a similar disinhibition and thus a more free and frank discussion? Are students more likely to admit that they don't know something and ask more questions? Is the interaction in SL likely to be healthier?
At least with the present technology and bandwidth it is difficult to provide simulations with enough richness and detail in SL. On the other hand the ability to voice chat and convey some emotions maybe sufficient to use SL as a medium for collaborative learning specially when participants are not all at one site.