We happen to use an 802.11n AirPort Extreme as our router, so we put today's new firmware upgrade to the test to see if TIme Machine really does work with AirPort Disk. As expected, it works and only requires a few simple steps...
First we updated our firmware using the AIrport Base Station Utility, then we plugged in our external HDD.
Next, we logged into our AIrport Extreme using Finder, clicked on the folder representing our external disk and finally, we fired up Time Machine.
After that, it was smooth sailing. It should be noted that Airport Express never supported AIrport Disk, and therefore doesn't work with TIme Machine.