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Today we are rolling out Android 5.1 - an update to Lollipop that improves stability and performance and offers a few new features like support for multiple SIM cards, Device Protection and high definition (HD) voice on compatible phones.

I haven't seen an official list or cleaned-up changelog* yet, but Android Police has a running tally under the "Feature Spotlight" tag: http://www.androidpolice.com/tags/feature-spotlight/Is there a full change list?