Apple: Time Machine via AirPort Disk unsupported
Monday, April 07, 2008 - 03:35 PM EDT"Apple confirmed for me last week that a feature for using hard drives attached via USB to an AirPort Extreme Base Station is an unsupported feature. The company declined to provide further information. This feature was available in Leopard betas, as has been widely reported, but was removed from the public Leopard feature list and from the shipping version. Apple had been working on providing me a definitive statement since my review of Time Capsule for Macworld was published on March 21," Glenn Fleishman reports for TidBITS.
"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 getting reports that USB-attached drives work erratically with an AirPort Extreme," Fleishman reports.
Full article here.
MacDailyNews Take: If it's not supported, then it shouldn't be possible to select the drive in Time Machine Prefs. Apple may have made itself liable for any data loss users may incur.
If Time Capsule is supported then a USB drive attached via Airport Extreme Base Station should also be supported.

Anything to get you to buy a Time Capsule instead.