The Drobo Storage Robot seems like an interesting device. In a nutshell, 4 bays for SATA HDs of any size. 2 or more HDs for data redundancy, and upgrade at will. Say 2 500GB HDs to start, fill the spare bays with 1GB HDs when necessary, then gradually upgrade those 2 500GB HDs one by one with 1.5GB or 2GB HDs. Rinse and repeat.
Common complaint is that the Drobo has no Firewire interface, but from the reviews this thing is no speed daemon so the USB 2.0 interface isn’t the bottleneck in any case. But then I like the Firewire interface because it is a lot easier on the CPU then USB 2.0.
One thing to note is that because the Drobo only supports Windows NTFS and Mac OS/X HFS+, unlike dumb HD casings, it might not work with those intelligent broadband routers as network storage. It also does not solve the “backup” problem. You would still need a gigantic HD or another Drobo for backup.