The event will be held at Nintendo's booth before the show floor opens, and will provide the media with the opportunity to check out the Big N's games before the mad rush ensues.
“This year, Nintendo is taking a different approach to E3. We’re giving you the chance to get closer to the games through a unique hands-on experience at our booth before the show opens,” the company stated.
At this point, we know that a new 3D Mario, Mario Kart, and Smash Bros. will all be at the expo in some form or another. It's also safe to assume the chronically-delayed Pikmin 3 will be there as well.