A couple weeks ago Google let slip that the Android 5.1 were running on Android One device. However there is no short press release about that or even an update for the Nexus line. As usual the Nexus line get the new Android versions first. But this was until today. It seems that Google is testing Android 5.1 on one Nexus device, the LG Nexus 5.
One of these devices was spotted at Geekbench running the latest Android version. So probably Google it’s planning an Android 5.a big released, possibly starting with the Nexus devices.
This it’s probably only a matter of weeks before we see it on the Nexus line. For other devices, the update to this version will depend on the companies that made them.