First Xbox One system update arrives

Xbox One's first required system update is out now, according to Microsoft's Larry Hryb.
Hryb revealed on his Major Nelson blog that the update was set to arrive tonight at 8 PM Eastern US time -- several hours ago, at time of writing.
The patch includes the following fixes and improvements:

• Addresses SmartGlass issues for some users when coming in and out of connected standby
• Addresses multiplayer issues for some users when re-joining games
• Addresses issues with inconsistent notifications for some users
• Addresses dashboard performance for some users
• Offers improvements for Xbox One's TV, system update, and content update services for scaling over time
• Updated wireless networking driver to improve connectivity issues for some users

Xbox One systems utilizing the console's "instant on" feature will automatically download the update in the background, install, and then power down. The hardware will apply the update the next time it's powered on, then reboot.
Owners who have disabled the aforementioned feature will be required to download and apply the update after this Thursday, December 12th.
Hyrb also added that the installation process should end up taking between two and five minutes.