It could actually be detremental for Nintendo to blow their wad and have Revolution playable. You need people to be saying that playing with the Revolution is an amazing expereince - not a mediocre one (which will most likely be the response with an early build). First impressions are killer. Once people find out that playing the Revolution is mediocre- there's no way to turn back. Then there'd be no more information to share, only spit and polish which will be added, but which won't make headlines.
What they do need is to reaveal the basic control scheme, and show someone (or a video of someone) playing games on the revolution using that new control scheme. Also, they need to reveal that the graphics will actually be good and comparible to PS2/Xbox, reveal that the console will not in fact be purple, and maybe reveal the identity of a single big launch game (I'm thinking Mario or Metroid).
And, if it plays well, and if Revolution is genuinly different than the other two consoles, and not gimmicky but actually revolutionary- it will take the spotlight right off the competition. It will make the headlines - on mainstream newspapers as well as videogame publications. And when CNN reports that 'NINTENDO REVEALS GAMES WILL BE CONTROLLED WITH MOTION IN NEXT-GENERATION CONSOLE' And, as a smaller headline, they report 'Sony and Microsoft unveil next consoles - greater graphics capabilty impresses gamers', we Nintendo fans will laugh and cheer.