Update 9 pm ET: Our comrade David Chartier from Ars Technica points out that the 7.1.3 firmware itself may not be necessary for the new functionality to work; he says he tested a 7.1 AEBS with a machine running Time Capsule & Airport 1.0, and Time Machine was able to see the remote disk. Other readers have reminded us that the disk must be formatted as HFS+ with journaling, and you may have to mount it in the Finder before Time Machine sees it. The freeware TimeMachineScheduler is disabled by this update, comments note.
Update 6:45 am Thursday: More comments point out that if you take a locally-connected Time Machine drive and attach it to an AEBS, you will be starting over with new backups (because the remote backups are stored on sparseimages, not as folders). Something to keep in mind if you already have a long backup history -- you might want to use a different drive.
Sometimes the fixes are subtle and quiet. Once TUAW reader Peder downloaded today's Airport updates and ran the utility, he noticed a new version of the Airport Extreme firmware queued up and ready (v7.3.1). When he installed and rebooted his AEBS -- which happened to have a USB hard drive hanging off of it... well, let him tell you:
After downloading the latest Airport-update I checked for updates for my AirPort Extreme. After upgrading to version 7.3.1, Time Machine recognised the attached USB-drive.
If this is a reproducible result -- this means you, everyone, go ahead and start testing this firmware! -- that means that the now-you-see-it, now-you-don't Time Machine to AirDisk feature of Leopard has finally arrived. Sure, the Time Capsule is a one-piece solution and quite economical, but for all the AEBS owners out there who have been waiting patiently, this would be a very nice bit of March madness indeed. [Response to "just-a-guy" below: Remember, this is the Airport EXTREME only; the Express doesn't support AirDisk at all.]
Seeing the same results as Peder? By all means let us know. He was kind enough to send us a few screenshots, see below.