X8 chipset is a modified Snapdragon S4 Pro with a beefed up GPU and two companion cores by Qualcomm. Qualcomm's top-of-the-line Snapdragon 800, which powers most new Android flagship devices, has Voice Activation built in to the chipset. Theoretically, this would mean that any Snapdragon 800 device would be able to leverage an ultra-low power mode to handle the always-listening voice commands. But, the question would remain as to if this update would be limited to devices running Android 4.4.The
If the functionality is already supported by the Snapdragon 800 chipset, it may be possible for Google to simply push an update to the Google Search app in the Play Store to add the functionality to any device with that chipset running Android 4.1 or higher (and we don't know of any Snapdragon 800 device that isn't at least on Android 4.2.) It may not happen right away, but it may be possible a little while after the Android 4.4 launch.
There has been evidence found both in the Google Search APK to support this idea, and the guys at TuttoAndroid did confirm that the "OK Google Now" command did work from the homescreen in their testing of Android 4.4 (although they didn't confirm if it worked when the device was in sleep mode.)
The rumors keep claiming that Google will announce the Nexus 5 and Android 4.4 any day now, so we should be getting the real story soon.