It's not in the US just yet, but Japan will receive the option to buy a standalone 250GB hard drive for the Xbox 360 starting March 11. Sadly, it'll run the equivalent of $US170.
This news comes shortly after the 120GB hard drive has turned up out of stock at most retailers, meaning the 250GB version could (and probably will) end up replacing the 120GB version. That is, if you despise your checking account. [Impress via PCWorld via Engadget]