You'll only get tomorrow's Xbox One firmware update if you manually choose to download accept it. But it will also enable all future console updates, including the Party-Chat-upgrade one in March, to be updated in the background if background updates are enabled, an Xbox rep told Kotaku. A good improvement!

UPDATE: For clarity's sake, I should have written "accept" instead of "download", as the Microsoft rep told me that users will have to manually "accept" this first one. It may auto-download, it may not. We'll find out very soon. Either way, it's a one-time situation.