Samsung and HTC have just published the Android 4.4 kernel source code and framework support files for their Google Play edition devices. While the code released today, a requirement of the GPL, won't be of any use to regular users, it does mean the rollout of KitKat to both Google Play edition phones is likely a matter of days away, if that. (The previous Android 4.3 OTA followed within a day of the open-source bits being released.)
HTC has previously stated that its code has been ready for some time, and that the public release timing was to be determined by Google.
So if you've got a Google Play edition Galaxy S4 or HTC One, now might be a good time to start checking for updates. We'll let you know as soon as the new KitKat-based firmware start rolling out.