The Chromium issue tracker reveals a screenshot of a Nexus 5 running Android 4.4.3. The screenshot is posted in an issue related to the synchronisation UI in the Chrome browser for Android. Previously we already spotted builds indicating Android 4.4.3 on the Nexus 7.
Unfortunately both the issue and the screenshot both don’t reveal many details on the upcoming Android Kitkat release. It’s expected that new Android version will fix issues related to data connections, random reboots and USB debugging. Previously US telecom provider Sprint reported that the update was rolling but the report appeared to be inaccurate.