I thought i would chime in with my Time Machine/Airport Extreme/AirDisk experience. First off, i had never used Time Machine before the new firmware arrived. Currently my equipment is as follows: 15 Penryn MBP, White Penryn MB, Airport Extreme Gigabit (purchased Nov 07) and a 250 gig CompUSA Brand (WD inside) Silver Case USB 2.0 External Hard Drive.
To begin this testing session i made sure that my Airport Extreme and computer had the newest Firmware and Software installed. I then formatted the 250 gig hard drive as OS X Extended with journaling using a GUID partition map. I gave it a very simple name of TM. I setup the drive to share with the Disk password and im not using Bonjour.
With both computer i have the newest airport software but i also installed the airport disk utility that i ripped out of the last Airport release DMG so i could have auto mounting of AirDisk's when they become available. Once performing this little addition i tested and everything worked great on both computers. They were able to both see the disk and mount it. I even tested going to sleep and coming back. It kept the disk mounted and i experienced none of the dropouts i used to experience when having an AirDisk plugged in.
On both of the computers, with the disks mounted, i fired up Time Machine for the first time and clicked on the change drive. Both computers saw the AirDisk and allowed me to use it as a Time Machine Backup Volume. They both then proceeded to begin backing up. I know this isnt a precise measure but i timed the transfer (55 gig for one, 22 gig for the other) and it was transfering a 100MB ever 5 seconds or about 20MB a second. Like i said, it was exact, just used finders transfer status. It seemed to be doing fine and i left them both up and running when i went to bed. When i woke up, both of them had finished the first backup.
So far i havent experienced any drop outs, both computers can see the drive and the sparse bundles they created on there. I am still able to access the drive and write other files to it as well. My next test will be to add another hard drive via a hub and see if i can share two disks, one for backup, the other for file storage. If this works successfully i will update everyone on the progress. So far though...everything has worked great....how i wished it would have worked back in November when i purchased it.