Storage Toolkit
Story: Apple Time Capsule: a first look
You CAN add additional drives...
Just like the newer version of Airport Extreme, you can use the USB port on Time Capsule to attach a printer or hard drive for sharing, and from the Time Capsule product page, "And if you want to share both a printer and an additional hard drive, you can. Just connect a USB hub to Time Capsule." The newer Airport Extreme says you can attach multiple printers and multiple USB hard drives via a hub, so I would imagine Time Capsule also supports that.
It's possible that Time Capsule will only allow backups to its built-in drive, but I doubt that. A more likely concern is that I don't think Time Machine, in it's current implementation, will span backup drives. That is, you may be able to see and backup to the additional shared drives, but you might have to manually move the backup location for some computers to one of the additional drives when space starts getting tight rather than just having it seamlessly continue using the additional storage on Time Capsule's attached USB drives when space on the internal one fills up. Still, that seems like a pretty easy thing to change in software at some point... and I suppose we could be pleasantly surprised and have it already support that (though it's more likely that we'll see that feature when Apple releases additional expansion drives in the same form factor as a new product :) ).
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