9to5Mac ran a quick piece this morning about hacking the new AirPort Extreme. Apparently Apple kept the hard drive caddy in place for the new AirPort Time Capsule, but left out any internal connectors.
In the worst ‘case’ scenario, DIYers could use the USB port and wire a bus-powered 2.5 inch hard drive (up to 2TB currently) or SSD around inside the case.
Don't do this. You don't really save a lot of money (if any by the time you paid shipping for a new 2TB HDD and USB compatible cable), and then you'd be using up an available USB slot that could be used for other things, like even more storage or a USB printer. If you want a hard drive, just pay more upfront for the AirPort Time Capsule.