Microsoft has no plans to release one stateside.
"Microsoft is now offering a standalone 250GB Hard Drive in Japan to provide consumers in Japan more storage options for their games and entertainment," a company spokesperson told IGN. "We work closely with retailers to determine the best approach for our products on a region-by-region basis."
If the recent 120GB harddrive shortage is any indication, it would seem the release of the 250GB harddrive in North America is coming shortly. Keep in mind, Xbox 360 harddrives are region free, so if you're looking for more space immediately, feel free to import one this coming March for 15,540 yen ($173).