I have been spending a large amount of time talking to people about interoperability in virtual worlds. One problem is defining what it means - and why it is important. From what I can tell, the main thing people are looking for is a way to easily traverse from one virtual world to another. It would be great to be able to jump through a portal from Second Life into Qwaq and back. We already support URLs as links between Qwaq spaces. Adding a Second Life island would be a bit tricky, but I think it could be done.
There is also mention of having one's avatar go along for the ride, but it turns out that this is not that important, and may not even be a good idea. One way to think about it is people already have multiple identities online. I have a work email address, personal email, and a number of random older email addresses that I still get mail from. I don't like mixing them, and I have different expectations for the kind of mail I get from them, and people have different expectations from me depending on whether it is a work or personal email. That doesn't even begin to address the difference in security requirements.
Avatars are context specific artifacts. My avatar in World of Warcraft makes no sense in my business world. An obfuscated name model like Second Life makes no sense in business either - good business practice is built upon a strong trust relationship. If I don't even know who you are I can't do business with you.
On the other hand, if you like how your avatar looks in world A, it would be nice to have a similar version in world B. I like having the same name for my email addresses at gmail and at Qwaq, even if my role in each is different.