We've asked Instagram about the nature of the test (or indeed, whether or not it's a test at all). If it's an experiment, though, it's a fairly broad one -- at least a few Engadget editors have it. This also appears to be a server-side addition, since people using regular Instagram releases have seen this show up.

That Instagram would try this isn't at all surprising. If you've stopped scrolling for long enough to stare at a photo, you probably want to do more than like it. Also, a feature like this could help Instagram continue its runaway growth. The easier it is to comment, the more likely it is that you'll get compliments and other interactions that could encourage you to stick around.

Update: Instagram says it started rolling out the feature on iOS a month ago, and that it's just reaching Android this week.