Google has already uploaded factory images to several Nexus devices and was in the process of pushing the source code to AOSP last night. Hopefully more information related to the 5.1 update is provided by OEMs in the coming weeks.
I've got three nexus devices ready for the OTA (or I'll just flash it latter today - was hoping google would ping me today & save me the trouble).
Can't wait to see what might be coming to our S5's soon (along with Samsung's tweeks of course).
There are some additional new capabilities made possible with the update, including "crystal-clear" HD Voice calling between phones running Android 5.1 and serviced by T-Mobile and Verizon
Well I know Sprint has their own HD voice available on certain phones calling to each other in the Sprint network so I guess it works the other way with Verizon and T Mobile.