It should come as no surprise that Android is due for a v5.1.1 release at some point, even if only to fix a particularly well-known memory leak. Now, thanks to an SDK update, we've got good reason to anticipate the new version is probably due out in the very near future. If developers check the SDK Manager for updates, they should see a new revision to the Platform SDK with a product description that reads "Android SDK Platform 5.1.1."
Now that the 5.1.1 version number is out there, it stands to reason Google has finalized the code and is either running some final tests or preparing for a release. AOSP still hasn't been updated with the new release, but we can probably expect that to change very soon. Once a version is finalized, new source code and firmwares usually come out in a fairly short window of time. Hopefully, this will also prompt the release of updates for the overlooked Nexus devices that still haven't received 5.1.
Note: Yes, this revision did come out on April 1st, which means it could possibly be a prank. However, since it's still around today, and it would be an abnormally subtle joke compared to most of the things we've seen on AFD, we're assuming this is legitimate.