When Microsoft announced that a bundle including two controllers, a new game and a 360 with a 250GB HDD was "only" $399, it led many to think the days of over-priced 360 HDDs was over. It's not.
In this little demonstration video for the new console (consoles need trailers now?), Microsoft's Larry Hryb has announced that the 250GB HDD won't be sold separately.
And really, this move isn't that surprising. The 120GB HDD retails for a ridiculous $150, but if the 250GB was ever released individually, it'd either be $300+, or would drive the prices of Microsoft's other HDDs down. Neither of which are obviously that appealing for you/MS.