All that I have read indicates that it will be like the "A Musical Journey" DVD that was bundled with the ROTS soundtrack, only performed live.

A narrator will tell what happens. Then there will be live music, performed by a symphony orchestra and a choir, while segments from the movie is shown on a big screen in the background. Then there will be some narration again, followed by another music+movie segment, and so on... Duration: 2 hours. The story of all six films is told.

I find it unlikely that Ian McDiarmid is going to be the narrator this time, or that John Williams himself will be conducting. We can only hope.

More links with info:
http://news.bbc.co.uk/1/hi/entertainment/7779011.stm
http://www.starwars.com/fans/events/news20081212.html
http://www.theo2.co.uk/general/the-o2-presents-star-wars-a-musical-journey.html

Tickets are out now, for the event on London's O2 arena. It is suppose to go on tour over Europe later. We'll see.

For those who have not heard the Star Wars score live: I was lucky to be able to go to a concert with John Williams' music earlier this year, and I found it to be awesome! Too bad though that the ticket price this time will start at £30 and go up to £100. The event I was on was considerably cheaper.