What does this icon mean, in the stock messenger app? Pretty straightforward; when viewing the list of recent contacts/conversations, the stock messenger app on my Samsung Galaxy S3 shows this icon, but for only the one conversation.

I don't know if it's a button, or notification, but I'm afraid to touch it.

Can anyone explain what it is/does?
Well, I believe I've found it.

As per this thread (for example) on the interwebs, it has to do with the Tango app, which I've recently installed.

Essentially, since Tango integrates with your phone like a parasite, when a contact is not available to use Tango the (x) icon indicates this in the stock SMS app.

Having cross-referenced this against similar threads, it seems to be the answer.