Google has announced on its official twitter and Google+ account that it will finally start rolling out Android 4.4 KitKat for Nexus 7 and Nexus 10 tablets today. The update will be available over-the-air and your device will automatically notify you once the update arrives.
The update is coming to Nexus 7 & 10 with wi-fi version only
The company has also revealed that remaining devices with cellular data and our beloved Nexus 4 will get the update soon. The exact date has not been specified but it should be arriving soon.
To remind you, Android 4.4 brings several improvements and UI changes including cloud print support, revamped email and downloads apps and more over the 4.3.