Is there a way to power down an AirPort Time Capsule gracefully?
According to Apple (from the Time Capsule manual)

Using Your AirPort Time Capsule Safely


The only way to shut off power completely to your AirPort Time
Capsule is to disconnect it from the power source.


If you want to go above and beyond, you can be sure that all users all logged off, and not performing a backup through the Airport Utility, but this is not necessary...