Apple came out with a fireware update for the AirPort Extreme Basestation and Time Capsule. People have reported being able to use exernal USB drives on the AEBS as a Time Machine destination. Great, six days after I got a Time Capsule for backups! 
I would still shy away from using the AEBS with Time Machine until there is a body of evidence that it works without fault. Some people have reported network drops and even kernel panics, and Apple is famous for not having the best disk support on the AEBS.
