AEBS + USB Hard Drive = Time Capsule
Well, it seems that today's release of firmware 7.3.1 for the AirPort Extreme Base Station (AEBS) now allows you to use Leopard's Time Machine feature with a USB-connected external hard drive. In effect, an AEBS + a USB hard drive = a Time Capsule.
Now that this change has been made, I will see about testing Time Machine/Time Capsule functionality between my MBA and my AEBS/USBHD over the Internet and report my findings here. As I've already indicated, though, Time Machine functionality may require a "fatter" bandwidth connection than is normally available over the "thin" Internet, so be prepared for me to confirm that using Time Machine across the Internet is not a good idea.
Either way, though, I will update the Guide entry from this thread with whatever information I find.