Apple updated their AirPort Extreme and Time Capsule. Now both have dual-band Wi-Fi.

From TidBits:
With Tuesday’s update to the AirPort Extreme Base Station and Time Capsule, you no longer need to make a choice or connect two different base stations. The revised models offer simultaneous dual-band networking at the same price as previous models: $179 for the Gigabit Ethernet AirPort Extreme, $299 for a Time Capsule with a 500 GB drive, and $499 for a 1 TB Time Capsule.
You can find out more about dual-band wi-fi at Apple’s website. Both are available now at the Apple Online Store.