Got a Nexus 6? Are you using it on Sprint? Then don't be surprised if you see an over-the-air update come sometime in the next few days. Sprint's support site had officially listed build LMY47Z, indicating that it's either going out to Nexus 6 owners now or will be in the very near future. The existence of this particular build was leaked last week.
Don't get too excited. According to the documents the update contains "general updates to Android" and "security enhancements." There's no mention of a version bump, but that does sound like most of what's included in Android 5.1.1, which went out to T-Mobile N6 units recently. We'll see when the first upgrades go through.
As always with post-Lollipop updates, any modifications to the software - no matter how small - will cause the flash to fail. If you've tinkered with the stock ROM and want to get on the latest version, you'll need to wait for the factory image to become available. Of course, since it's a Nexus phone, you can also track down the more generic version of 5.1.1 and flash that instead.