I don't see the point of Apple even putting a disk inside the Time Capsule box. Why not just have a FW800 port and let users buy their own disk? Then if you like, hook up a Drobo unit that is stuffed with 1.5 TB SATA drives. Disk drive technology moves quickly.
Again like the HP smartmedia server Drobo is a different market.
Time capsule is the play it in and leave it alone solution.
Drobo is the "it grows with you" solution plus Drobo make the server that can support two drobo devices already where would the value be in competing with them.