We haven’t seen an official announcement anywhere, but the beta signup process works and will serve a newer Google app than the normal channel. Presumably Google will use this the same way it does the Chrome beta channel—to test in-development features and bug fixes. We signed up, and our Google app updated from 18.104.22.168 to 22.214.171.124, but it doesn’t seem like anything significant has changed.
Google could potentially make a lot of changes through the Google app. It doesn’t just cover Google searches on Android; the Google app is the home screen, the app drawer, Google Now and Now on Tap, voice recognition, and the always-on voice commands.