Just like the web version of Google+, the Android app will now show you at least some of your friends' "+1" activity in the main social stream. The new functionality (which has more than a little in common with the way Facebook does things) should allow Google+ to be even more social - you'll be able to weigh in on more of your friends' activity, or at least the activity that doesn't include full posts. The updated app went live yesterday.
The tiny changelog reads as follows: "Discover posts that your friends have +1'd." Google stresses that people who have added you to their circles won't be able to see anything they couldn't before just by clicking on your profile - the public and private options for new posts are intact. If you want to change who can see your +1 activity (or at least who will see it in their stream) head to this settings menu and click the "Edit" button.
Of course, this will increase the volume of your primary stream based on how many people are in your circles, and not everyone is thrilled about that. But if Google+ is to step up to its rivals in engagement as well as raw user numbers, this sort of thing is essential. The functionality should be expanding to iOS before too long.