When the new Xbox 360 S was released, it was shortly noted that the console's hard drive was different from the previous models, having a much slimmer and sleeker profile. With the announcement of the new "Arcade" Xbox 360 S model, which features only 4GB of internal memory, those wanting more storage space would be forced to pick up the premium model --- but this should be changing very soon according to new press materials released by the company.
In the latest images of the packaging for the Xbox 360 S "Arcade" model, it notes that the 250GB HD can be purchased separately. Those who frequent retail stores will note that the hard drive isn't currently being sold, so this is an indication that Microsoft is planning to sell them separately in the near future, likely to be timed alongside the new model's release.
Microsoft has yet to comment or official announce the hard drive's existence, and it's currently unknown if it will be marked up like the previous proprietary Xbox 360 hard drives were.