Mythos, maybe. Altough I am not sure not announcing at CES = losing momentum.
I think showing the system, specs, and some games all in development at E3--just blow away E3--with the promise, "You can own one in November 2005" would be fine. If they release specs now that gives Sony 5 months to find ways to play down Xenon pluses, point out Xenon negatives, and to empahsis PS3 pluses. And if necessary, they could even alter bits and peices from now till May.
I would guess that offically anouncing the system and some movie demos at GDC, and then at E3 release specs, release date, and playable demos would be a good ONE-TWO. They build the hype of "its coming" with some great video feeds and the promise of playable demos at E3. Then at E3 steal all the thunder. Playable units, game demos, and specs. Sony might (will?) have a more powerful machine, but it does not give Sony a chance to pick apart the Xenon specs. A console with running games goes a lot farther than a promised system.
So I think this was a good move by MS. Revealing your hand too early can be bad. MS has some great momentum from this holiday season, unless Sony/Nintendo beat MS to the punch before E3 I do not see them losing that momentum.