An often overlooked feature of the Airport Extreme is it's ability to work as a Network Attached Storage (NAS) device. That's fancy speak for a simple file server on your local network.
It's not the most sophisticated solution as far as these things go, but it's certainly all that's required for most people, especially for basic tasks like sharing a disk for backups, whether using Time Machine or SuperDuper to a disk image. You can manage sharing disks with a password for access to all disks or setup individual user accounts, each with their own password.
One issue with the Airport Extreme is that there's only one USB port, so if you want to share multiple disks or a printer at the same time you need to get a USB hub which means yet another power plug taken up and more unsightly cables messing things up.
I've been happily using the Minipartner disks for my IDE drives for a number of years, but these disks only work over Firewire, even though they integrate a USB hub for simplifying the connection of multiple USB devices. The latest generation are more flexible and have the option of presenting the internal SATA disks over Firewire or USB, and they still have the integrated USB hub. This makes them absolutely ideal for use with the Airport Extreme since you can easily daisy chain several disks using the USB connections and they're exactly the same shape as the base station so it's an elegant stack of devices, especially when you use the very short USB cables to attach each disk to the next.
With these disks you have all of the advantages of simple daisy-chaining like you used to do with Firewire over a USB connection and you can share multiple drives from the Airport Extreme without an extra external USB hub in the mix.
Now if only I could arrange the disks to format them into RAID groups directly from the Airport drive so that it could manage larger volumes than a single disk at a time and it would be the perfect inexpensive home NAS solution.
Of course, a Drobo hooked up to the Airport Extreme is the ideal solution, but the entry price is a little steep, and I still want to see one that handles more than 4 disks - maybe an 8 drive system that uses 2.5" disks?
That way I'd lose less to the parity calculations on the RAID so eight 500Gb drives would give back 3.5Tb of useful space instead of 3Tb from four 1Tb disks. Not to mention that I seem to be collecting 2.5" SATA disks faster than 3.5" ones.
Update [4-nov-2008]: Just noticed that the Airport Extreme is smart enough to put attached disks to sleep if they're not being solicited across the network.