Earlier this week, I dropped a hint that Android 5.1 could be released this week. It hasn't happened yet, but 5.1 is so close, I can almost smell it. 

The Nexus download page https://developers.google.com/android/nexus/images was updated yesterday afternoon, along with the drivers page (Last updated March 6, 2015.)

T-Mobile did the same to Nexus 4, Nexus 7 2013, and Nexus 5 pages (like this one https://support.t-mobile.com/docs/DOC-7404).

Also, check out this reference https://code.google.com/p/android/issues/detail?id=60359#c46 (thanks, +Chris Lacy):

the other half of the fix is in the system image;
api 22 has the fix and it is on the way to release.
api 21, 19, 18 will be updated with this and other fixes soon.

Perhaps it was pushed to early next week... Apple's Watch event is on Monday - what could be a better way to steal some of the fanfare than to release a major OS revision on the same day?