The results to my survey question, "What is the most effective and engaging way to hold an SL event featuring the avatar of a real world celebrity?", are in, and frankly, they're about what I hoped. A plurarity chose "Depends on the celebrity", and that's pretty much where I stand. If it's a singer who plans on performing during her SL appearance, a la Suzanne Vega, then of course you should use VOIP; if it's a writer like Julian Dibbell or a public figure known for his writing, as with Judge Posner, then just as surely you want them communicating via text, through which they can shine best. (Actually, I personally lean slightly toward the "unplugged" route of standard chat.)
I understand the reasons for going with VOIP, so what I'm about to say isn't meant as a criticism of others who opt for it with their real world celebrity appearances. All else being equal, however, VOIP in SL has a long way to go before it becomes truly immersive and uniquely worthwhile to the world, in my opinion. Location-based audio and convincing real time lip synch would be a start, as would an intuitive menu of voice-triggered animations and poses, or better yet, a way of automatically animating those through speech recognition. Until then, we're still dealing with a real world celebrity who's only half there, as it were, a zombie animated by someone else and whose disembodied voice emerges from everywhere except their mouth. I also worry that there's a vaguely undemocratic feel to the process-- leaving the impression (if occasionally unfair) that the rich and famous only deign to appear in-world when they can rely on additional, external elements, and not even bothering with the communication tools the majority of Residents use.