Nintendo's GameCube message at SpaceWorld 2000 was compelling and adequate. They showed the hardware, gave specs, and showed several "tech demo" videos. Granted, they weren't competing for attention since it was a Nintendo-only show, but I think that much would be enough.
Live demos and show floor playable games aren't necessary just yet. The games are far too early along anyway. Anything along the lines of a live Miyamoto demo would just be a huge bonus to me.
As for the competition stealing their ideas, the dev kits have GOT to come out shortly after E3. If Nintendo doesn't share the big secret at E3, MS and Sony will find out almost right away anyway because the secret will leak when the kits ship. Any fool should be able to assume this.
(As an aside editorial, being "different" is not as important as Nintendo thinks it is. So even if the idea is copied, their lunch isn't going to be eaten just because of that. Being "different" didn't work for GameCube. Why would it for Revolution?)
Also remember that Nintendo's comments on what they will or won't show were made BEFORE Sony and MS got into last week's pissing contest. The situation has changed. Nintendo may just be laying in wait to see what Sony and MS do first. I think they'll be capable of showing a lot, but they're not going to volunteer more than they have to.
All that being said, Nintendo has 3 platforms they need to cover in their presentation. You know there will be an extensive Zelda demo. You know there will be a wireless DS demo. Unfortunately, Revolution will likely be the last and lowest priority item, because at the end the the day, they need to make a case for their current lineup of products. But I am hopeful they'll impress us somehow anyway.