Update April 14: Nexus 7 (2013) WiFi and LTE models, along with the Nexus 4 are now available.
Update April 8: Google added Android 5.1 (LMY47D) images for the Nexus 7 2012 “Mobile.”
Updated March 25: Google added Android 5.1 (LMY47I) images for the Nexus 5 and Nexus 6.
Updated March 20: Google added the Android 5.1 (LMY47D) image for the Nexus Player.
Updated March 17: Google just added a new Android 5.1 factory image for the Nexus 6 as build LMY47E.
Updated March 12: Nexus 6 factory images added.
The first batch of Android 5.1 factory images are now available for the Nexus 5, Nexus 10, and Nexus 7 (2012).
You will find links to all below. We will update this post as we have more.
Factory Images (Android 5.1):
Instructions for flashing factory images can be found here.