If you want to use the Time Machine feature in Mac OS X Leopard to set up automatic backups, you'll have to find a networking cable.
In the advance marketing material for Leopard, which goes on sale later today, Apple had promoted the ability to connect an external USB hard drive to an AirPort Extreme wireless router (Airport Disk) and use Time Machine to wirelessly back up the hard drives of Macbooks scattered around a home. Time Machine, one of the more universally liked features in Leopard (click for CNET's review), is designed to make backing up and restoring files much easier than before.
But Macworld's Dan Frakes (who has an excellent spin move to the basket) noticed earlier this week that Apple had pulled the language advertising the use of an AirPort Disk from the Time Machine promotional copy. AppleInsider then verified that its copy of Leopard did not allow the use of an Airport Disk to do wireless backups, forcing you to physically connect the USB drive to a Macbook or Macbook Pro to make sure the backup operation takes place. Time Machine can also back up data to another Mac running Leopard that is set up for file sharing, a server running Leopard, or one of Apple's Xsan storage devices, but casual Mac users are much more likely to back up to one of the many external hard drives available.
That's certainly not the end of the world, but it's somewhat inconvenient if you're used to using your notebook on a wireless network at all times. Posters on AppleInsider's forums speculated that backing up an entire hard disk over a wireless network would take forever, even at 802.11n speeds of over 100Mbps, and that seems to make sense. But certainly Apple was ready to go forward with the feature up until the last minute, so it must have been convinced as recently as a few weeks ago that wireless backups wouldn't be an issue.
The most likely scenario seems to be that Apple pulled it from the final release of Leopard at the last minute because the software simply wasn't ready. The company can add support for Airport Disks at a later date with a software update, but Leopard early adopters will have to wait.