I'm afraid I haven't enough experience and projects yet to be able to make assertions regarding the adoption of a different identity and how students feel about it. Thus far, all of the students like seeing the use of avatars inside their online language courses, but my colleague and I who are most interested in their use have not experimented sufficiently with asking the students to adopt a different identity. We usually ask the students to create an avatar that best represents who they feel they are rather than to create an alternate persona. We are just now working in second life ourselves to see what kinds of projects we may potentially develop.
I personally feel that any "tool" used in second language learning is only part of the process and that if used appropriately, I see a great potential for avatar use to aid us, but I don't necessarily feel as if it should be used exclusively to bring about learning and communication. I work extremely hard to develop the teacher-student and student-student relationships in the online courses which to me is the ultimate key to success and I am open to exploring the myriad ways we have available to us in order to make that happen!
Thank you for your comment, and I apologize that I cannot be more helpful--I still have much to learn.