The Nexus 6 is the first Nexus phone to have an official home on all four big US carriers. That means there are bound to be some carrier-specific updates, even for a Nexus phone. T-Mobile recently hinted that 5.1.1 was on the way with WiFi calling, and now we've got a leak from Sprint that says 5.1.1 (LMY47Z) will be out on May 26th.
The changelog claims general Android updates and security enhancements (i.e all the minor stuff in 5.1.1 vs. 5.1). The software will probably be more or less the same as what is delivered to the T-Mobile variant, but with a different build number and some minor tweaks. Of course, these are Nexus phones, so you can flash to a different build of 5.1.1 if you want.
This isn't official yet, but we have every reason to believe the source is legit. So keep an eye out next week.