Apple confirmed for me last week that a feature for using Leopard's Time Machine backup with hard drives attached via USB to an AirPort Extreme Base Station is unsupported. The company declined to provide further information. This particular networked backup option was available in the betas of Mac OS X 10.5 Leopard, as has been widely reported, but was removed from the public Leopard feature list and from the shipping version of the operating system. Apple had been working on providing me a definitive statement since my review of Time Capsule for Macworld was published on 21-Mar-08. (Normal file sharing is a fully supported feature, however, as it has been since the AirPort Extreme with 802.11n was released in early 2007.)
What a "lack of support" means is that if you attempt to use an 802.11n AirPort Extreme Base Station for Time Machine backup, you won't get any help from Apple's technical support, something that readers have already told me. I've been receiving reports that USB-attached drives work erratically with an AirPort Extreme. TidBITS editor Joe Kissell and I have been discussing the strange array of scenarios in which you find an Apple Filing Protocol (AFP) volume that's shared by the AirPort Extreme server not appearing automatically for Time Machine. (See "MacVoices Podcast Covers Time Capsule Ins and Outs," 2008-04-03, for links to the podcasts. I also talked about Time Capsule and this problem in a podcast on 26-Mar-08 with Jason Snell, editorial director for Macworld.)
This option to choose an AirPort Extreme-connected drive first appeared with the release of several related firmware, driver, and operating system updates on 19-Mar-08 (see "AirPort Update Extends Time Capsule, Adds AirDisk Support," 2008-03-19). I speculated at the time that this addition was an error on Apple's part, perhaps due to a debugging feature left turned on that wasn't properly turned off before the updates shipped. This was buttressed in part by the way in which Time Capsule drives - whether an internal drive or ones connected externally via USB - appear via Bonjour in a list of selectable volumes when setting up Time Machine, but AirPort Extreme disks do not.
For more background on this situation, see the original response I had back on 2008-01-17, "Time Capsule and Its Associated Rage Factor," and details on Time Capsule's USB drive support in "Time Capsule Ships with Support for USB Drive Backups," 2008-02-29.
Somewhere along the line, Apple changed the name of this concept of sharing drives from USB over the network from AirDisks to AirPort Disks.