Google has begun the rolling out process for it's first devices to receive the latest 4.4 - Kitkat, Android update which are the three tablets namely the Nexus 7 2012, Nexus 7 2013 and the Nexus 10; after the Nexus 5 was launched with 4.4 on Halloween. The news was confirmed by the Android twitter team. Currently it should be noted that only the Wi-Fi only versions of the devices are receiving the update.
The next devices reported to receive the KitKat update will be the SIM versions of the Tablets and the 2012 Nexus 4.
If you're not an owner of a Nexus device, then an update for your device may still be some way off. Currently only HTC and Motorola (also owned by Google) have officially disclosed that they are working on an update for their devices. Other schedules for leading android phone manufacturers such as Sony, Samsung etc are not yet revealed by their respective companies.
Although the update for the Nexus Wi-fi only tablets have been rolled out, it is rolled in phases - so It may require some time before you receive a notification to upgrade, and it may even take upto a week.
I myself have a Nexus 7, which I will review as soon as it receives Android 4.4 - so stay tuned!