Microsoft is be working on a new 100GB HDD for the Xbox 360:
The announcement came at the Korean version of X06, a Microsoft-specific event in the Eastern part of the world that has more of an international focus than other big events like TGS and E3, so it's unknown exactly to what markets outside of Korea this peripheral is headed, but it would be unusual for Microsoft not to eventually bring it over to the US. Reportedly, it will be making its global release in March in Korea and potentially other countries.
The current Xbox 360 HDD has a capacity of 20GB. More info at TG Daily.