Apple released Mac Pro SMC Update 2.0 on Wednesday, a firmware update for the company's new Mac Pro (2013) that allows the device to run in Power Nap mode without using the fan for most Power Nap activities.
Naked Mac Pro (2013)
Apple's release notes in full:
This update is recommended for all Mac Pro (Late 2013) models. This update enables Mac Pro to enter Power Nap without running the fan for most Power Nap activities, and addresses a rare issue where a low-speed USB device may not be detected at boot.
You can download the firmware update from the Mac App Store. As of this writing, Apple hasn't added this update to its Downloads site.