I've cancelled the order for the Airport Extreme and instead bought a 500GB Time Capsule. The main reason is that Apple still doesn't officially support external disks on the AEBS for Time Machine, yet they seem to for Time Capsule.

The other idea I had was to remove the included drive and replace it with a cable to connect an external RAID1 box via SATA. I've found the parts I need (an SATA gender changer and SATA to eSATA cable), but I haven't read about anyone else doing this yet. Obviously this mod wouldn't be officially supported and would likely void the warranty.