I was alerted to the Ant and Dec Twitter interview right before it started - the two television hosts were using the Waterstone's Twitter profile to answer questions from fans. Not too long ago, Canongate helped to organise the first rock Twitter interview, where Dave Simpson and a number of ex-members of The Fall also fielded questions from fans and indulged in some general chat.
I think it's safe to say that having your interview subjects in the same place where the more technically-minded can assist and using the same Twitter profile (as Ant and Dec did) makes for a more organised Twitter chat. The chaos of The Fallen Twitter interview was incredibly fun (and tiring!) and we learned that having multiple Twitter profiles answering a multitude of questions from new and experienced Twitterers is extremely challenging to follow and (later) transcribe.
As for publicity, everything's about social media these days (or at least it seems to be). What's great about Twitter and other real-time public online communication projects is that it gives us the ability to try things out at minimal financial cost. Sometimes they work, sometimes they don't, and I think Waterstone's may have shown one way that Twitter interviews can work (assuming your interviewees can be in the same place at the same time and at a godly hour, too).
Now, all I need to do is convince the boss that it's important for all our authors (The Mighty Boosh, Nick Cave, David Simon...) to log onto our Twitter profile - with me present, of course - and answer reader / fan questions.