Google posts Android 5.1 factory image for Nexus 5, OG Nexus 7 Wi-Fi and Nexus 10
The factory image also comes in handy if you've rooted your Nexus device and need to flash the latest official build so that you don't miss an OTA update. Any reason you might have for wanting to return to stock Android is a reason to employ the factory image.
As we told you this afternoon, Android 5.1 supports the use of Dual SIM cards, allowing multiple users to share a single phone. Security is improved as lost or stolen devices remain unlocked until the rightful owner signs in with his Google credentials. Even a factory reset won't change that. And Android 5.1 also brings HD Voice. Both sides of the conversation must be using a compatible phone, and both must use a carrier that supports the feature.
More Nexus devices will soon get the Android 5.1 factory image, and we will pass this news along to you as it happens.
source: Google via AndroidCentral