Unfortunately, it's only for T-Mobile. Over the weekend, we reported that T-Mobile and Google had finally gotten WiFi Calling working on the Nexus 6, and that the update - which is Android 5.1.1 - would be rolling out over the weekend to a few people and then a more proper rollout this week. Well the rollout is still going, but now we have our factory image. So those on T-Mobile can go ahead and flash the Android 5.1.1 update to their Nexus 6. Google does say it's T-Mobile only, which is a bit confusing seeing as there are only two variants, one for the US and one for the rest of the world.
The entire factory image is about 896MB. And the build is LYZ28E. If you are waiting on the OTA, it'll be much smaller, probably under 100MB, but seeing as this is the entire factory image, it's a much larger file. The update includes WiFi Calling for T-Mobile as well as various other bug fixes to improve the Nexus 6.
The Nexus 6 is actually the last Nexus device to receive the Android 5.1.1 update. As the Nexus 9, Nexus 7, Nexus 5, Nexus 4, and Nexus Player have all received their Android 5.1.1 update. So the delay was likely due to T-Mobile trying to get WiFi Calling going, which was actually a feature that T-Mobile and Google announced along with the Nexus 6 last fall with Android 5.0 Lollipop.
It's surprising to see Android 5.1.1 getting its factory image today for the Nexus 6, when we are just days away from Google I/O. Google I/O takes place this Thursday and Friday from Moscone Center in San Francisco, and we will be there to see exactly what Google has to show off. We already know that Android M is making an appearance, and we should be getting another developer preview Thursday evening. Which we've heard will focus on RAM and battery, which are just a few of the many issues that have plagued Lollipop users over the past few months. We should see new Nexus hardware this fall though along with the final release of Android M, whatever they decide to call it. My bet is on Macadamia Nut.