Apparently Microsoft didn't have time during their admittedly busy keynote to announce the new 256MB Memory Unit. Lucky for us, a Word doc on their site confirms that such a beast exists:
"With four times more capacity than the original, gamers can easily store and transfer even more Xbox 360 saved games, Xbox Live profiles, full Xbox Live Arcade titles and other entertainment content downloaded from Xbox Live Marketplace."
It's unclear if this model will be replacing the currently shipping 64MB MU, or is simply an additional, larger, and most likely more expensive, alternative for space hungry gamers.
Read - Xbox.com's Xbox 360 Accessories (.doc)