Hopefully Apple doesn't make up some way to disable this in the future, but it seems that despite claims elsewhere on the web, the latest Airport Extreme can be used with a USB hard drive to perform Time Machine backups. I've always heard great things about the Airport devices and really wanted to do backups over it, but I already had a larger USB hard drive, so a Time Capsule seemed like a bit of a waste. This worked with an Airport Extreme 802.11n 5th generation. My Macs are both Lion and Snow Leopard.
All you have to do is first plug the hard drive directory into your Mac, partition as desired and format filesystems as an HFS (Mac OS Extended). Then, plugin the drive and enable disk sharing on the Airport. After that's done, your Aiport should show up in Finder. Once you mount it, you can select it from the Time Machine settings (it will appear as a Time Capsule).
That's it! Glad I found out and wasn't deterred by what Apple says and is on their forums. The Airport has been amazing, video streaming is far smoother now unlike with the aging wireless router I was replacing. Definitely a recommended buy!