If you have a hard drive connected to your AirPort Base Station with 802.11n and are running OS X 10.5 Leopard, you should be overjoyed that you can now use AirDisk with Time Machine!
All you have to do is select Software Update from the Apple Menu and install the AirPort updates. You may need to open AirPort Utility from your Utilities folder and select Check For Updates from the AirPort Utility menu to get the latest firmware as well.
Once the updates are installed, you need only open Time Machine from System Preferences and point it towards your AirDisk. This does essentially the same thing as a Time Capsule, except with Time Capsule you get an enterprise-quality hard drive and the base station in the same shiny white box.