This would be a major mistake. Gamers and press would of course be super excited and euphoric - but Nintendo would not have any surprise or big announcement for E3. And they need some mind-blowing stuff there to make sure that not everyone will just talk about PS720.
My guess for today is that they will mainly show off already announced games and services (Pikmin, TW101, Virtual Console, Google Maps, Panorama View, etc.). They may show one or two games which we don't officially know yet (Yoshi's Land, etc.). I don't think that we will see anything from Monolith, EAD Tokyo or Retro today. Smash Bros. or Animal Crossing would be the best thing which could possibly happen.