Interesting little application that provides full R/W access to 360 HD and memory units. They mention the first possible use thrown around is sharing savegames over the net (like here: http://www.360gamesaves.com/) but the possibilities are really endless.
http://www.myxbox360.com/news/news/Xplorer360__Full_Read_Write_Access_to_HDD_MS_700/
http://www.myxbox360.com/news/news/Xplorer360__Full_Read_Write_Access_to_HDD_MS_700/